Duality constraints on supersymmetric unified models and radiative symmetry breaking

Motivated from unified models with string origin, we analyse the constraints from duality invariance on effective supergravity models with intermediate gauge symmetry. Requiring vanishing vacuum energy and invariance of the superpotential couplings, the modular weights are subject to various constraints. Further, the intermediate gauge symmetry breaking scale MU is related to the values of modular weights of the matter and Higgs fields. For certain regions of values of the latter, MU can be close to the conventional unification scale of the minimal supersymmetric standard model. We also examine particular examples where the intermediate gauge symmetry breaks down to the standard gauge group radiatively.