Exploratory Spectroscopy of Magnetic Cataclysmic Variables Candidates and Other Variable Objects^*

Its CRTS light curve ranges from 16.5 to 19.5 mag on timescales of days. The spectrum is typical of polars with Hβ more intense than Hα. The He ii 4686 Å emission line is as intense as Hβ (Table 2) and the high-ionization Bowen C iii/N iii is also present. In our follow-up observational study, Silva et al. (2015) confirmed MLS0227+13 as an eclipsing polar with an orbital period of 3.787 hr, using polarimetric, photometric, and spectroscopic data. The system has no X-ray counterpart, which can be explained by its large distance from us (Silva et al. 2015).

CSS0357+10 was detected as a transient by CRTS on 2009 November 9. The V magnitudes are highly variable, spanning from 16.5 to 21 mag in short timescales. Schwope & Thinius (2012) performed a time-resolved photometric study of this target and found a periodicity of 114 minutes with 0.8 mag amplitude, interpreted as the orbital period of a non-eclipsing polar candidate. Recently, Thorstensen et al. (2016) obtained a spectroscopic time series of this source and corroborated the 114 minutes orbital period. This object is also detected in X-rays (ROSAT, XMM-Newton), presenting large variability (Schwope & Thinius 2012).

The identification spectrum is dominated by intense Balmer and He ii 4686 Å lines. It also presents He i and weak He ii 5411 Å, Bowen C iii/N iii complex and Fe ii 5170 Å emissions. The Balmer and He ii 4686 Å lines are asymmetric, with extended red wings, and Hβ is more intense than Hα (Table 2), confirming the polar classification.

This object was detected as a variable star on 2013 December 10 by the CRTS. Its light curve spans from 18.5 to 21.5 mag, reaching 1 mag variation amplitude in timescales of hours. Narrow Balmer emission lines dominate the identification spectrum, which also presents He i and He ii 4686 Å lines. The latter line consists of a broad component (EW = 18 Å) superimposed on a very narrow central emission (EW = 1 Å). The composite line profile, the EW of Hβ, and the ratio between He ii 4686 Å and Hβ (Table 2) suggest a mCV classification. The continuum shows humps that may be associated to cyclotron harmonics. This property and the absence of an X-ray counterpart are consistent with a low accretion-rate polar or a pre-polar system (e.g., Schmidt et al. 2005). The power spectrum of the CRTS data has its highest peak at 0.09851004 days (2.36 hr), which has an FAP smaller than 0.0001 (=0.01%). The formal error of this period is $1.3\,\times \,{10}^{-7}$ days (see Section 4). The corresponding phase diagram (Figure 2) is compatible with the light curve of a polar system. This period places MLS0456+18 inside the period gap of CVs. As several objects in this work, a definite classification demands more observations.

The CRTS light curve of MLS0720+17 presents 0.5 mag amplitude variability around 18.5 mag, occasionally dropping to 20–21 mag, possible during eclipse events (Drake et al. 2014). The Goodman identification spectrum is typical of polars, with Balmer, He ii 4686 Å, C iii/N iii, and He i narrow emission lines superimposed on a continuum with a blue slope. The He i line profiles are asymmetric with extended blue wings in this spectrum. No X-ray source was found near its coordinates. The time series analysis applied to the CRTS data does not show any periodic signal that could explain the excursions to lower fluxes. Hardy et al. (2017) have recently presented a light curve spanning approximately 1h. It shows a modulation that the authors interpret as an eclipse. We consider that it can arise from cyclotron emission, as usually seen in polars. Moreover, the narrow spectral lines are inconsistent with an eclipsing system having a disk.

This target matches a blue stellar counterpart in the SDSS, with u = 20.18, g = 20.58, r = 20.43, i = 20.28, and z = 20.06 in DR8. It presents two brightness levels with averages of 19.7 and 21.5 mag in the CRTS data. It is associated with the X-ray source 1WGA J0854.0+1336 in the WGA Catalog of ROSAT Point Sources.¹¹ This object is also classified as BOSS J085402.10+133632.8, a z = 2 QSO in the MILLIQUAS Catalog V4.7,¹² with no SDSS spectrum available. However, our spectrum is completely consistent with a Galactic object, being similar to the spectrum of a polar in the low accretion state, with cyclotron humps and emission lines of He ii 4686 Å, Hβ, and Hα on a blue continuum. No periodic signal was found in the CRTS time series analysis.

This source was discovered by ROSAT (Beuermann & Thomas 1993) and classified as a polar with an X-ray orbital period of 106 minutes (Beuermann & Schwope 1994). 1RXS1002-19 was observed with XMM-Newton (Ramsay & Cropper 2003) and the resulting X-ray light curve suggests that the source of radiation is located in one hemisphere of the WD, while modeling of the X-ray spectrum yields a WD mass close to 0.5 ${M}_{\odot }$.

Our optical spectrum of 1RXS1002-19, the first for this source, has a strong blue continuum. The He ii 4686 Å line is more intense than Hβ, which in turn is more intense than Hα (Table 2). He i features are also present in emission. The FWHM of the Hβ and He ii 4686 Å lines are around 2000 km s⁻¹ (Table 2), which is unusual for a polar. The equivalent width of the Hβ emission is 11 Å, smaller than the value of 20 Å from the Silber's criterion (Silber 1992) for a magnetic CV classification. Our spectrum converts to V = 18 mag. This locates the object in the polar systems region of X-ray count rate versus optical mag diagram, according to Figure 2 of Beuermann & Thomas (1993).

CSS1127-05 has a blue point source counterpart in the SDSS, with $u=20.00,g=20.12,r=20.09,i=20.28$, and z = 20.15 in DR8. The CRTS light curve clearly shows two distinct brightness states with average magnitudes of 18.5 and 20.5. The exploratory spectrum has a high signal-to-noise ratio, displaying strong and narrow Balmer lines in emission on a flat continuum, besides less intense He ii 4686 Å and He i. Hβ is more intense than Hα. The Hα EW (204 Å) is the largest among the objects we classify as CV (Table 2). In spite of the low ratio between He ii 4686 Å and Hβ (0.16), considering the Silber's criterion (Silber 1992), we suggest that it is a polar due to the presence of high- and low-brightness states. However, we do not discard a possible IP classification since these objects may also present low states, though less commonly. No significant periodicity was found in the CRTS data. The system has no X-ray counterpart.

The light curve of CSS1503-22 presents two alternating brightness states at average magnitudes of 19.5 and 17.5, both states with dispersion of 1 to 1.5 mag. Woudt et al. (2012) performed photometric monitoring of CSS1503-22 in 2010 March and April on five runs summing up 15 hr of observations when it was at high state. They found a period of 133.38 minutes and, from its photometric behavior and the detection as a Swift X-ray source, they suggested it to be a polar. The flux of our SOAR spectrum corresponds to 19.7 mag, which is consistent with the CRTS low state. The spectrum presents absorption features from 4500 to 6000 Å, which we interpret as from the stellar components. Some absorptions occur at typical wavelengths of TiO bands from a M2–M5 secondary star (see, for instance, Mennickent et al. 2002 and Gray & Corbally 2009). The absorptions near Hβ and to the blue of Hα are probably due to Zeeman splitting in the photosphere of the white dwarf. See, for instance, the SDSS J214930.74-072812.0 spectrum and the superimposed model atmosphere with B = 44.7 MG presented in Külebi et al. (2009). The spectrum also displays narrow Balmer lines, possibly with blue wings, and Hβ being half the intensity of Hα (Table 2). The He i 5876 Å line is also present. The Zeeman absorptions hamper the detection of possible He ii 4686 Å or Bowen complex emissions. The EW of Hβ is 16 Å, slightly smaller than the value of 20 Å from the Silber's criteria (Silber 1992), but the accuracy of the measured EW can be affected by the cited absorptions.

The CRTS light curve of this object has measurements scattered from 16.8 to 20.5 mag. The exploratory spectrum presents intense and narrow Balmer lines in emission. These lines are slightly asymmetrical, presenting extended red wings. Hβ is more intense than Hα, while He ii 4686 Å is almost as intense as Hβ (Table 2). The Bowen complex at 4630–4650 Å is clearly visible. The spectrum shown in Figure 1 is the average of three consecutive spectra, obtained with 20 minute intervals. They show a remarkable variability in the continuum, hence they are individually presented in Figure 3. We performed a 3.6 hr span photometric time-series of this source on 2014 April 28, using the 1.6 m P&E telescope at OPD and the Andor iKon-L936BV camera, obtaining 39 images with a 240 s exposure time through the V filter. The differential photometry was converted to V magnitude using as reference the star NOMAD1 0793-0295201, with m_v = 16.51 (Zacharias et al. 2004).

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The OPD light curve is consistent with a polar classification and shows a modulation with a period around 0.077 days and 0.60 ± 0.05 mag amplitude. In the search for periodic signals in the CRTS data, the PDM method yielded a period of P = 0.075439 ± 0.000001 days, compatible with the OPD period. Figure 4 shows both OPD and CRTS light curves folded with P = 0.075439 days and a epoch of ${T}_{\min }=2\,456\,776.7108$ HJD. The average magnitude of the OPD data is V = 21.3; therefore, the source was caught in a very low state, whereas the individual spectra correspond to 17.8 and 19 V mag. The system has no X-ray counterpart.

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This CV was discovered by Denisenko & Sokolovsky (2011) among X-ray sources in the ROSAT catalog. Denisenko & Martinelli (2012) performed time-resolved photometry and suggested the target to be a polar or IP, based on the amplitude (0.8 mag in white light) and shape of the light curve. They determined an orbital period of 2.08 hr.

Our exploratory spectrum is typical of polars, with very intense He ii 4686 Å and Hβ more intense than Hα. The C iii/N iii complex is also present and the continuum increases to the blue. Our spectrum corresponds to V = 16.3 mag.

The CRTS light curve of this object presents two brightness levels separated by 2 mag and a short timescale variability. Coppejans et al. (2014) performed photometric monitoring during the bright state and classified it as a polar from its light curve, which is modulated with a period of 1.53 hr. Our spectrum was obtained in the low-brightness state (V ≈ 19 mag) and presents narrow, single peaked Balmer, He i and He ii 4686 Å emission features over a blue continuum. The Balmer and He ii lines have extended red wings and Hβ is 35% more intense than Hα (Table 2). The C iii/N iii Bowen complex at 4640 Å and the lines of Fe ii 5169 Å and He ii 5412 Å are also visible. The system has no X-ray counterpart.

This source presents many kinds of variability in the CRTS light curve: long- (years) and short- (one month) term variations between magnitudes 18 and 15, and 1 mag variations in timescales of a few days. SSS1956-60 is classified as the nova-like system CV V348 Pav (Downes et al. 2001) and is also identified as V1956-6034, a polar candidate by its spectrum (Drissen et al. 1994). Our exploratory spectrum displays narrow Balmer, He i, and He ii emission lines. The line of He ii 4686 Å is almost as intense as Hβ, which in turn is more intense than Hα (Table 2). Also present in the spectrum are the lines of the Bowen complex, He ii 4541 and 5412 Å and Fe ii 5169 Å. In the spectrum obtained in 1992 by Drissen et al. (1994), the ratio of the intensities Hβ/He ii 4686 Å is 0.25, much smaller than the value of 0.91 we measured in our spectrum from 2012. An X-ray counterpart was detected with a 1.8 counts s⁻¹ total band count rate at these coordinates by XMM-Newton in the slew data mode, and was registered as XMMSL1 J195647.7-603425 in the XMM-Newton Slew Survey Clean Source Catalog (Saxton et al. 2008).

The discovery of this polar candidate is reported by Kryachko et al. (2010). From the R light curve, they obtained an orbital period of 0.0848 days (2.035 hr), and their spectrum shows He ii 4686 Å almost as intense as Hβ and also He i and the Bowen blend. Afanasiev et al. (2015) performed polarimetric observations of this source and found variable circular polarization reaching $-31 \%$.

We obtained exploratory spectra of USNO0825-18 in two occasions in 2012, 35 days apart. In the second run, the flux in the blue was six times more intense than in the first observation (Figure 1) and the He ii 4686 Å to Hβ intensity ratio increased from 0.97 to 1.23 (Table 2).

This object was discovered and classified as an eclipsing CV by CRTS on 2009 October 9 in a single outburst when it reached V = 17.2 mag. CRTS data span from 2005 to 2015, during which the object showed average V = 19 mag and ∼1 mag variability in timescales of hours, with occasional 21 mag excursions. Another outburst was recorded by ASAS at 16.39 mag on 2013 August 27 (Ohshima 2013). Szkody et al. (2014) present a spectrum with strong emission Balmer lines obtained on 2013 September 3 and classify the object as a dwarf nova, possibly eclipsing due to the dispersion of the CRTS quiescent light curve. This spectrum was taken in a state brighter than the quiescent flux in the CRTS light curve. The SDSS DR9 counterpart of CSS0104-03 presents u = 19.2, g = 19.5, r = 19.1, i = 18.8, and z = 18.9.

Our Goodman spectrum shows narrow and single peaked Balmer, He i, and He ii 4686 Å emission lines, Hβ being 1.26 times more intense than Hα (see Table 2). The continuum is flat with a slight slope to the blue. The Balmer lines have extended wings and are slightly asymmetrical. The average continuum flux in our spectrum is a factor of four less intense than the continuum flux in the spectrum published by Szkody et al. (2014), in which the line of He ii 4686 Å is only marginally detected. CSS0104-03 has the highest He ii 4686 Å EW among our sample of disk-accretion systems. Our spectrum has a flux level around 19 mag in the V band, hence it is consistent with the mean level of the CRTS light curve.

Our time series analysis of the CRTS data does not reveal any period that could explain the low flux excursions as eclipses. The small widths of the lines are also inconsistent with an eclipsing system. Thus the system could be a near face-on dwarf nova. However, this classification does not explain the low flux measurements, the two-component emission lines, and the He ii 4686 Å line behavior, since its intensity was expected to be higher at the spectrum from Szkody et al. (2014) taken at a brighter state. The object has no X-ray counterpart. If we suppose that its low optical brightness is due to its distance, it is possible that its X-ray luminosity is not high enough to allow the object detection in X-ray surveys. This seems to be true for MLS0227+13 (Silva et al. 2015). Therefore, a magnetic classification is a reasonable possibility taking into account the observations. Moreover, if the brightest points correspond to eruptions, it could be an IP system, possibly similar to GK Per or EX Hya. Spectroscopic time series would be useful for a definite classification of this system.

The light curve of this source, obtained by CRTS, has a large amplitude of variation, from 18.5 to 20 mag. It might have originated from flips between two brightness states. The average spectrum presents broad emission lines of Hα and Hβ. Hα shows a profile with two peaks, while Hβ is quite peculiar, with a strong central absorption at the rest position of the line, in the center of the broad emission. Also marginally detectable are absorption lines of the Na i doublet at 5890–5896 Å and the diffuse interstellar band (DIB) at 6284 Å. Our time series analysis does not reveal any remarkable signal. This object has no evidence of being a magnetic system.

This CRTS source has been discovered by SSS on 2011 February 3, receiving the ID SSS110203:101217-182411 and was later detected by CSS, on 2012 March 24, when it was assigned as CSS120324:101217-182411. The CRTS light curves varies between 18 and 20 mag with sporadic outbursts reaching 16–17 mag. Our spectrum displays a blue continuum and broad He i and Balmer emission lines, Hβ being more intense than Hα. He ii 4686 Å is present as well. The spectrum flux corresponds to approximately 19.3 mag in the V band, consistent with the quiescent flux level. Coppejans et al. (2016) classify it as a dwarf nova with three detected outbursts. Its spectrum resembles the spectrum of the IP candidate CTCV2056-30 (see Section 6.2.11) and short-period IPs like SDSS J2333 (Szkody et al. 2005; Southworth et al. 2007) and HT Cam (Kemp et al. 2002), so we suggest that it is a member of this class. The String-Length spectrum of the CRTS data shows some structure, but they do not translate into discernible features in the folded diagrams.

This object is associated with the X-ray source 1RXS J101239.6-325839. The CRTS light curve has short-term (days) variability between 17.3 and 19.5 mag, but no periodicity was found. Moreover, the CRTS light curve does not present DN-type eruptions. The spectrum has low S/N, but Hα is clearly seen as a broad emission line, while Hβ is weak. The continuum is quite blue. Despite not showing spectral features characteristic of magnetic systems, its ROSAT colors match those sources. The spectrum shows modulations that might be due to absorptions features of a red stellar component, but we could not associate their wavelengths to features usually found in CV secondaries. We do not classify this system as a mCV because of the lack of spectral evidence.

SSS1359-39 was discovered by CRTS and its light curve has a quiescent brightness level at 19 mag with a dispersion of 1 mag, and sporadic outbursts at magnitudes 14–15. The periodicities search had negative results. This source is associated with the X-ray source 1RXS J135915.6-391447. The exploratory spectrum shows features related to an accretion disk, with intense and broad Balmer emission lines (FWHM > 1000 km s⁻¹—Table 2) along with He i, Fe i, Fe ii, and He ii 4686 Å, and is very similar to the spectrum of the IP candidate CTCV2056-30 (Section 6.2.11), so we classify it as a possible short-period IP.

Photometric monitoring from CRTS yields a flat light curve at 17 mag with rapid and short occasional decreases of 3 magnitudes. SDSS registers this object as a blue point source with $u=21.85,g=21.88,r=21.42,i=21.38$, and z = 21.25. Drake et al. (2014) obtained a spectrum of this object in the high-accretion state, with broad Balmer absorptions on a blue continuum. Our SOAR spectrum was obtained in a faint state (V ≈ 20.2 mag) and shows the Balmer lines in emission. These lines are broad with double peak profiles, indicating a possible high orbital inclination. The system may be eclipsing, which would explain the rapid and deep drops in the CRTS light curve. The time series analysis of the CRTS data does not reveal any clear periodicity, probably because most of the fainter points are upper limits and are not included in the photometric CRTS data. If the object is indeed eclipsing, our spectrum (exposure time of 2700 s) would have been taken predominantly during the eclipse, which should be relatively long. This source could be alternatively interpreted as a VY Scl nova-like variable after its spectral and photometric behavior.

The light curve obtained by CRTS is nearly flat around 17 and 17.5 mag with one episodic measurement at 14.3 mag and another at 18 mag. Thorstensen & Skinner (2012) obtained one spectrum in 2011 September in which Hα is visible in emission together with a K-star feature at 5168 Å. Kato et al. (2013) found two photometric periods (0.05436 days and 0.05827 days) but the interpretation of them is still not clear. The authors suggested they could be related to superhumps. Spectral observations by Drake et al. (2014) reveal CSS1735+15 in eruption with a steep blue continuum and absorption features of Hγ and Hδ. The SOAR spectrum has the same flux level as the spectrum from Thorstensen & Skinner (2012) and displays typical features of a K star and Na i absorption together with emission lines of H i and He i. Our spectrum does not have enough coverage to allow luminosity class determination, so we cannot discard that the secondary is an evolved object. After the submission of this manuscript, Thorstensen et al. (2016) presented time-series spectroscopy and determined an orbital period of 8.48 hr and a distance of 1830 ± 330 pc from a K4 ± 1 spectral-type secondary star. This large period challenges the interpretation of the modulations as superhumps, which occurs for systems below the period gap. CSS1735+15 is unlikely an mCV.

This is another CV discovered by Denisenko & Sokolovsky (2011) among X-ray sources in the ROSAT catalog. It was classified as a possible dwarf nova by its X-ray hardness ratio. Our exploratory spectrum is reddened, with emission lines of H i, He i and marginal evidence of He ii 4686 Å, besides two DIBs at 5780 and 6284 Å. The feature at 5577 Å is associated with bad subtraction of the telluric [O i] line. This CV has no evidence of being a magnetic system.

MLS2043-19 was discovered as a transient optical source by CRTS. Its light curve has a quiescent level at ∼19.5 mag and four registered outbursts, reaching 15 mag. The CRTS data do not present any evidence of eclipse or periodicity. The SOAR spectrum, on the other hand, presents broad and double-peaked Balmer and He i emission lines, which suggest a high orbital inclination system, besides weak He ii 4686 Å or C iii/N iii emission features over a blue continuum. It also reminds us of the spectrum of the IP candidate CTCV2056-30 presented in Section 6.2.11, so we suggest it is a short-period IP candidate.

This object, discovered by CRTS, has a bimodal light curve with a low-brightness state at 19.5 mag with 1 mag variation and several outbursts reaching 18–16 mag. Coppejans et al. (2014) found a photometric period of 0.09598 days, suggesting it to be associated to superhumps of an SU UMa type CV, and estimated an orbital period of 0.0917 days. Our exploratory spectrum shows double-peaked Hβ emission more intense than Hα, besides He i and He ii 4686 Å (and possibly He ii 4542 Å), being similar to the spectrum of the IP candidate CTCV2056-30, which suggests possible magnetic accretion. Interestingly, if the photometric period was the orbital period, the object would be in the period gap of non-magnetic CVs.

This object was discovered by Augusteijn et al. (2010) during the Calán-Tololo Survey. Their low-resolution spectra present Balmer and He i emission lines over a blue continuum. The radial velocity curves provided a period of 1.76 hr assumed as the orbital period, while photometric monitoring yields an additional modulation with P = 15.4 minutes, leading to a highly probable IP classification. Augusteijn et al. (2010) also report some likely dwarf nova outbursts registered by ASAS and the ROSAT X-ray detection as 1RXS J205652.1-301433. Our spectrum shows Hβ as intense as Hα and relatively intense He i and weak He ii 4686 Å lines. The continuum is flatter and about three times less intense in the blue than the spectrum from Augusteijn et al. (2010). The spectrum is quite similar to that of short-period IPs like SDSS J2333 (Szkody et al. 2005; Southworth et al. 2007), HT Cam (Kemp et al. 2002), and DW Cnc (Patterson et al. 2004; Rodríguez-Gil et al. 2004), among others. These objects present spectra that are indistinguishable from quiescent dwarf novae spectra and, therefore, show that a single optical spectrum is not enough to confirm the IP nature of a source. The definite IP classification demands the detection of the white dwarf spin period not synchronized with the orbital period.

This system was discovered by CRTS in 2011 May, displaying a light curve with a quiescent level at 18 mag and outbursts reaching 15 mag. Coppejans et al. (2014) performed time-resolved photometry in 2011 October and found eclipses, deriving an orbital period of 0.15699 days. A period of 0.1569268 days was determined independently by Drake et al. (2014) from the photometric data in the CRTS itself, in both quiescence and outburst.

We obtained two exploratory spectra of CSS2108-03, in 2012 May 20 and 2012 June 25. The data display characteristic features of a disk system. Emission lines of H i and He i are broad and have pronounced double peak profiles, consistent with the high orbital inclination shown by the eclipses. The continuum flux measured in June was steeper to blue and five times larger than that obtained in May. We do not classify this object as an mCV.

This object was discovered by CRTS and suggested to be a polar type CV in the CRTS circulars.¹³ Its light curve presents a quiescent level with long-term (years) variability ranging from 18 to 19.5 mag and several outbursts reaching about 16.5 mag. de Budè (2011) performed photometric follow-up observations, which yield non-eclipsing light curves (within a maximum time span of 3.6 hr) dominated by flickering with less than 0.5 mag amplitude.

Our exploratory spectrum displays broad Balmer and He i emission lines with double peak profiles, which are more conspicuous in the He i lines. The presence of a disk, evidenced by the double peak lines and corroborated by the outbursts, discards the polar classification. He ii 4686 Å and C iii/N iii are weak but present. The continuum shows absorption features, which could be associated to a cool secondary. The spectral features are somewhat similar to the spectrum of CTCV2056-30. We classify it as an mCV candidate.

Also selected as a new CV from the ROSAT catalog, 1RXS2223+07 presents a DN light curve with outbursts 2 mag above the average 19.5 mag quiescent level (Denisenko & Sokolovsky 2011). Our spectrum presents broad Balmer and He i emission lines, besides He ii 4686 Å, on a slightly red continuum, consistent with a DN spectrum.

This source presents a CRTS light curve varying between 16.5 and 18 mag with two outbursts reaching 15–15.5 mag. Its position matches that of the X-ray source 1RXS J224150.4-662508. Our spectrum, obtained at OPD, shows emission lines of Hα, Hβ, He i, and He ii 4686 Å over a continuum that is slightly steeper to the blue. The spectrum is very similar to the spectrum of the IP candidate CTCV2056-30 described above which, as in this case, also presents outbursts and X-ray detection. These similarities may indicate an IP nature for SSS2242-66.

CSS2319+33 has a highly variable CRTS light curve with short and long timescales, ranging from 16.5 to 19.5 mag, but it does not show any outburst. It matches a blue source at SDSS DR8, with $u=17.45,g=17.79,r=17.68,i=17.36$ and z = 17.02 and also matches the ROSAT source 1RXS J231909.9+331544. Despite being noisier, the OPD spectrum is very similar to the spectrum of SSS2242-66, which in turn resembles the spectrum of CTCV2056-30, so we consider it as an IP candidate.

Denisenko & Sokolovsky (2011) identified this system as USNO-B1.0 0840-0137592, the optically variable counterpart of an X-ray source in the ROSAT catalog, and suggested a classification as a dwarf nova. Their analysis of archive photographic plates shows that most of the time the system is fainter than 17 mag in the B band, but occasionally gets brighter, reaching 16.5 mag. Our exploratory spectrum has a flux in the V band of ∼15.3 mag, brighter than in any previous detection, corroborating a dwarf nova classification. Our spectrum is typical of a dwarf nova in eruption, showing narrow Balmer and He ii 4686 Å emission lines superimposed on a very blue continuum. During DN eruptions, broad absorption troughs may develop at the base of very narrow Balmer emission cores, mainly in the higher member of the Balmer series—see Warner (1995) and the case of SS Cyg by Hessman et al. (1984). These features are also observed in 1RXS0721-05. Emission lines of He i, the interstellar absorptions lines of Na i 5890 and 5896 Å, and the diffuse interstellar band (DIB) at 6284 Å are also present in our spectrum. If we consider its ROSAT count rate (Boller et al. 2016) and the quiescent optical magnitude, the object is located in the dwarf novae region in Figure 2 of Beuermann & Thomas (1993). The ROSAT hardness ratios (HR1 = 1.000 ± 0.360 and HR2 = −0.108 ± 0.276) are also consistent with a dwarf nova classification (e.g., Hümmerich et al. 2014).

SSS2042-60 has a CRTS light curve with two distinct brightness levels at 18 and 19.5 mag, being observed most of the time at the higher state. No eruption was recorded. Our SOAR spectrum displays a very steep blue continuum with Hα in emission and Hβ in broad absorption with a weak central emission feature. He ii 4686 Å is marginally detected and Na i absorption is visible. Our spectrum has a flux equivalent to V = 18 mag, near the brightest state. We suggest that the object is a nova-like system, possibly of VY Scl type due to the fadings in its light curve.

This source was discovered by XMM-Newton in 2011 as a soft X-ray transient (2011 December 01) and was suggested to be a Nova because of its X-ray softness (Read et al. 2011a). Observations with GROND identified the optical counterpart with g' = 18.4 mag and r' = 19.5 mag, too faint for a recent Galactic Nova (Kann et al. 2011). The optical source has a blue SED, also atypical for a Nova, with a hint of strong He ii emission. Swift observations show a very soft X-ray spectrum with kT = 48 eV, 12 times fainter than the XMM-Newton flux obtained 19 days before (Read et al. 2011b). XMM0630-60 was also selected as a blazar candidate, but this classification was not confirmed (Cowperthwaite et al. 2013).

Our spectrum is compatible with a Nova in the nebular phase and displays strong blended lines of [O iii] 4959 and 5007 Å. Besides, broad and near rest position emission lines of He ii 4686 Å, Hβ and Hα are also present, with multiple narrow components. Our spectrum corresponds to V ≈ 21.8 mag.

This CRTS object varies between 20.5 and 18.5 mag and was suggested to be a polar type CV or an AGN.¹⁴ The exploratory spectrum is compatible with a Type I AGN at z = 1.257 based on the conspicuous broad emission line of Mg ii 2798 Å and numerous blended UV Fe ii multiplets.

CSS0315-01 shows a CRTS light curve slowly varying between V = 19 and V = 20.5 mag. A broad Mg ii 2798 Å line and the UV Fe ii multiplets in the spectrum lead to a classification as a Type I AGN at z = 0.8513, further confirmed by the detection of [Ne v] 3425 Å. [O ii] $\lambda \lambda$3727, 3729 Å are also detected.

This CRTS transient is highly variable in the range from 18.5 to 21 mag. The spectrum presents narrow emission lines of [O ii] 3726/3728 Å, Hγ, Hβ, and [O iii] 4959/5007 Å at a redshift z = 0.272, besides absorption lines of Ca ii H and K. An upper limit to the flux ratio [O iii]/Hβ is 0.39, which classifies CSS1240-15 as a H ii galaxy (Kewley et al. 2006), corroborated by the ratio [O ii]/[O iii] = 5.4 ± 0.6 (Baldwin et al. 1981). The large amplitude variability of this source, on the other hand, leads to a distinct classification as an AGN caught at a minimum state, with the central source completely hidden from direct view, or as a low excitation radio AGN (LERAG, Padovani et al. 2015). Follow-up spectroscopy is necessary to unveil the real nature of this object.

This source is highly variable in CRTS light curve, ranging from 16 to 22 mag in timescales of months. The exploratory spectrum corresponds to an AGN at z = 1.1887, determined from a emission line at 6120 Å, which we identify as Mg ii 2798 Å. Absorption doublets are clearly visible at 5700 Å and 5010 Å. These are Mg ii 2796,2803 Å absorption-line systems from two distinct intervening gaseous clouds in the line of sight of the AGN, at redshifts of ${z}_{1}=1.037$ and ${z}_{2}=0.791$, respectively. Absorption features like the doublet of Fe ii 2586,2600 Å, visible at 4640 Å, and the Mg i 2852 Å line, visible at 5108 Å, are also present and are associated with the gaseous structure at ${z}_{2}=0.791$. The strong variability of MLS1251-02 suggests that it is an optically violently variable (OVV) quasar.

The CRTS light curve of MLS1402-09 displays a low-brightness level at 20.5 average magnitude and a high level at 19.5–20 mag. The spectrum shows a prominent narrow emission line at 5493 Å, which we identify as [O ii] 3726/3728 Å and from which we derived z = 0.4736, and absorption lines of Ca ii H and K. In the NVSS catalog, there is a radio source (NVSS J140203-090747) offset by 4307 from MLS1402-09 with a measured flux of 3.2 ± 0.6 mJy at 1.4 GHz. This flux level classifies it as a low-emission radio AGN (LERAG, Padovani et al. 2015).

MLS1404-15 was suggested to be a CV by CRTS from its blue DSS color and high variability. The CRTS light curve has a low state at 19.5 average magnitude, changing to a high state at average 18.5 mag from 2009 on. A prominent broad feature centered at 5321 Å, that we associated to Mg ii 2798 Å at z = 0.9017, dominates the spectrum. We also identified the UV Fe ii multiplets at both sides of Mg ii 2798, typical of Type I AGNs. The presence of high-ionization lines of [Ne v] 3347,3427 Å and the permitted O iii line at 3134 Å give additional support that MLS1404-15 is indeed an AGN.

Classified as a possible variable star by CRTS, MLS2230-04 has a bimodal light curve with a low state at 20 mag and a high state at 19.3 mag. The SOAR spectrum is dominated by a broad Mg ii 2798 Å emission line and hints of UV Fe ii multiples at both sides of Mg ii, classifying this source as a Type I AGN at z = 0.968.

MLS1519-13, MLS2044-16, and MLS2132-15 were selected for their CRTS light-curve variability. The identification spectra are consistent with A or F spectral types, with narrow Balmer absorption lines over a blue continuum. The time-series analysis of the CRTS photometric data yielded periods of 0.521 days for MLS1519-13, 0.535 days for MLS2044-16 and 0.486 days for MLS2132-15. The CRTS light curves of these RR Lyrae stars, folded on the respective periods, are shown in Figure 5.

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SSS1448-40, SSS1639-23, and SSS1940-63 were also selected from the CRTS variability. The exploratory and fitted reference spectra for spectral-type estimation are presented in Figure 6. The DFT and Lomb–Scargle period search methods applied to the CRTS data yielded a period of 87 ± 1 days for SSS1448-40 and 307 ± 16 days for SSS1639-23, but no coherent period was obtained for SSS1940-63 (Figure 7).

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