ELECTRIC-MAGNETIC DUALITY AND SUPERSYMMETRY IN STRINGY BLACK-HOLES

We present a generalization of the U(1)2 charged dilaton black hole fa mily whose main feature is that both U(1) fields have electric and mag netic charges, the axion field still being trivial. We show the supers ymmetry of these solutions in the extreme case in which the correspond ing generalization of the Bogomolnyi bound is saturated and a naked si ngularity is on the verge of being visible to external observers. Then we study the action of a subset of the SL(2,R) group of electric-magn etic duality rotations that generates a nontrivial axion field on thos e solutions. This group of transformations is an exact symmetry of the N = 4, d = 4 ungauged supergravity equations of motion. It has been a rgued recently that it could be an exact symmetry of the full effectiv e string theory. The generalization of the Bogomolnyi bound is invaria nt under the full SL(2,R) and the solutions explicitly rotated are sho wn to be supersymmetric if the originals are. We conjecture that any S L(2,R) transformation will preserve supersymmetry.