Dilatonic black holes in a S-duality model

We find exact charged black hole solutions of a string effective action that is invariant under S-duality transformations. These black hole solutions have the same causal structure as the Reissner-Nordstrom (RN) solutions. They reduce to the RN solutions for self-dual configurations of the dilaton and to the Garfinkle-Horowitz-Strominger (GHS) solution in the weak (or strong) coupling regime. Using the purely magnetic solutions of the S-duality model we also generate dyonic black hole solutions of the GHS model, which have the causal structure of the RN solutions.