Dyonic BPS saturated black holes of heterotic string on a six-torus

Within effective heterotic superstring theory compactified on a six-torus we derive minimum energy (supersymmetric), static, spherically symmetric solutions,which are manifestly invariant under the target space O(6,22) and the strong-weak coupling SL(2) duality symmetries with 28 electric and 28 magnetic charges subject to one constraint. The class of solutions with a constant axion corresponds to dyonic configurations subject to two charge constraints, with purely electric (or purely magnetic) and dyonic configurations preserving 1/2 and 1/4 of N=4 supersymmetry, respectively. General dyonic configurations in this class have a space-time of extreme Reissner-Nordstrom black holes while configurations with more constrained charges have a null or a naked singularity.