String loop corrections to the universal hypermultiplet

We study loop corrections to the universal dilaton supermultiplet for type IIA strings compactified on Calabi–Yau 3-folds. We show that the corresponding quaternionic kinetic terms receive non-trivial one-loop contributions proportional to the Euler number of the Calabi–Yau manifold, while the higher-loop corrections can be absorbed by field redefinitions. The corrected metric is no longer Kähler. Our analysis implies in particular that the Calabi–Yau volume is renormalized by loop effects which are present even in higher orders, while there are also one-loop corrections to the Bianchi identities for the NS and RR field strengths.