  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.
