Three-Family Perturbative String Vacua: Flat Directions and Effective   Couplings
  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. 
Lisa Everett (Penn U.)