N=1 dual string pairs and gaugino condensation

We study a class of four-dimensional N=1 heterotic string theories which have non-trivial quantum dynamics arising from asymptotically free gauge groups. These models are obtained by orbifolding 4d N=2 heterotic/type II dual pairs by symmetries which leave unbroken products of non-abelian gauge groups (without charged matter) in a ''hidden sector'' on the heterotic side. Such models are expected to break supersymmetry through gaugino condensation in the hidden sector. We find a dual description of the effects of gaugino condensation on the type II side, where the corresponding superpotential arises at tree level. We speculate that the conformal field theory underlying the type II description may be related to a class of geometrical non-supersymmetric string compactifications. (orig.)