GENERAL-CLASS OF BPS SATURATED DYONIC BLACK-HOLES AS EXACT SUPERSTRING SOLUTIONS

We show that a four-parameter generating solution for a general class of four-dimensional, spherically-symmetric, static, dyonic BPS saturat ed solutions of leading-order effective equations of toroidally compac tified heterotic string theory is an exact string solution. The corres ponding ten-dimensional background defines a conformal sigma-model whi ch is a particular case of a 'chiral null model' with curved 'transver se' part. The exact conformal invariance is a consequence of the chira l null structure of the 'electric' part of the model and the N = 4 wor ld-sheet supersymmetry of its transverse 'magnetic' part. The sigma-mo del action has a remarkable covariance under both target space T-duali ty and the electro-magnetic S-duality transformations, and it illustra tes the relationship between string-string duality in six dimensions a nd S-duality in four dimensions. In general, there exists a large clas s of exact six-dimensional superstring solutions described by chiral n ull models with four-dimensional transverse parts represented by N = 4 supersymmetric sigma-models with metrics conformal to hyper-Kahler on es.