  The properties of a class of quasi-realistic three-family perturbative
heterotic string vacua are addressed. String models in this class generically
contain an anomalous U(1), such that the nonzero Fayet-Iliopoulos term triggers
certain fields to acquire string scale VEV's along flat directions. This vacuum
shift reduces the rank of the gauge group and generates effective mass terms
and effective trilinear interactions. Techniques are discussed which yield a
systematic classification of the flat directions of a given string model which
can be proven to be F- flat to all orders. The effective superpotential along
such flat directions can then be calculated to all orders in the string (genus)
expansion.
