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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-Nordstr\"om black holes while configurations with more constrained charges have a null or a naked singularity. 1996 The American Physical Society.
Cvetič et al. (Mon,) studied this question.