QUANTUM MODULI SPACES OF N=1 STRING THEORIES

Generically, string models with N=1 supersymmetry are not expected to have moduli beyond perturbation theory; stringy nonperturbative effect s as well as low energy field-theoretic phenomena such as gluino conde nsation will lift any flat directions. In this work, we describe model s where some subspace of the moduli space survives nonperturbatively. Discrete a symmetries forbid any inherently stringy effects, and dynam ical considerations control the field-theoretic effects. The surviving subspace is a space of high symmetry; the system is attracted to this subspace by a potential which we compute. Models of this type may be useful for considerations of duality and raise troubling cosmological questions about string theory. Our considerations also suggest a mecha nism for fixing the expectation value of the dilaton.