Constraint Supersymmetry Breaking and Non-Perturbative Effects in String   Theory
  We discuss supersymmetry breaking mechanism at the level of low energy N=1 effective superstring actions that exhibit $SL(2,Z)_T$ target space modular duality or $SL(2,Z)_S$ strong-weak coupling duality. The allowed superpotential forms use the assumption that the sourse of non-perturbative effects is not specified and as a result represent the most general parametrization of non-perturbative effects. We found that the allowed non-perturbative superpotential is severely constrained when we use the cusp forms of the modular group for its construction. By construction the poles of the superpotential are either inside the fundamental domain or beyond. We also found limits on the parameters of the superpotential by demanding that the truncated potential for the gaugino condensate never breaks down at finite values in the moduli space. The latter constitutes a criterion for avoiding poles in the fundamental domain. However, the potential in most of the cases avoids naturally singularities inside the fundamental domain, rendering the potential finite. The minimum values of the limits on the parameters in the superpotential may correspond to vacua with vanishing cosmological constant. 
Christos Kokorelis