  We consider axion-free quantum corrected black hole solutions in the context
of the heterotic S-T model with half the N=2, D=4 supersymmetries unbroken. We
express the perturbatively corrected entropy in terms of the electric and
magnetic charges in such a way, that target-space duality invariance is
manifest. We also discuss the microscopic origin of particular quantum black
hole configurations. We propose a microscopic interpretation in terms of a gas
of closed membranes for the instanton corrections to the entropy.
