MASSLESS BLACK-HOLES AND CONIFOLDS IN STRING THEORY

Low-energy effective field theories arising from Calabi-Yau string com pactifications are generically inconsistent or ill-defined at the clas sical level because of conifold singularities in the moduli space. It is shown, given a plausible assumption on the degeneracies of black ho le states, that for type II theories this inconsistency can be cured b y nonperturbative quantum effects: the singularities are resolved by t he appearance of massless Ramond-Ramond black holes. The Wilsonian eff ective action including these light black holes is smooth near the con ifold, and the singularity is reproduced when they are integrated out. In order for a quantum effect to cure a classical inconsistency, it c an not be suppressed by the usual string coupling g(s). It is shown ho w the required g(s) dependence arises as a result of the peculiar coup lings of Ramond-Ramond gauge fields to the dilaton.