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 an intermediate gauge symmetry. Requiring vanishing vacuum energy and invariance of the superpotential couplings, we find that modular weights of the matter and higgs fields are subject to various constraints. In addition, the vacuum expectation values of the higgs fields breaking the intermediate gauge group, are determined in terms of their modular weights and the moduli. We also examine the possibility of breaking the intermediate gauge symmetry radiatively.