Lawrence Berkeley National Laboratory

Starting from the previously constructed effective supergravity theory below the scale of $U(1)$ breaking in orbifold compactifications of the weakly coupled heterotic string, we study the effective theory below the scale of supersymmetry breaking by gaugino and matter condensation in a hidden sector. Questions we address include vacuum stability and the masses of the various moduli fields, including those associated with flat directions at the $U(1)$ breaking scale, and oftheir fermionic superpartners. The issue of soft supersymmetry-breaking masses in the observable sector presents a particularly serious challenge for this class of models.