hep-th/9504090 MASSLESS BLACK HOLES AND CONIFOLDS IN STRING THEORY

Low-energy eeective eld theories arising from Calabi-Yau string compactiications are generically inconsistent or ill-deened at the classical level because of conifold singularities in the moduli space. It is shown, given a plausible assumption on the degeneracies of black hole states, that for type II theories this inconsistency can be cured by nonperturbative quantum eeects: the singularities are resolved by the appearance of massless Ramond-Ramond black holes. The Wilsonian eeective action including these light black holes is smooth near the conifold, and the singularity is reproduced when they are integrated out. In order for a quantum eeect to cure a classical inconsistency, it can not be suppressed by the usual string coupling g s. It is shown how the required g s dependence arises as a result of the peculiar couplings of Ramond-Ramond gauge elds to the dilaton.