  We discuss supersymmetry breaking mechanisms at the level of low energy N=1
effective heterotic 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 source of non-perturbative
effects is not specified and as a result represent the most general
parametrization of non-perturbative effects. The minimum values of the limits
on the parameters in the superpotential may correspond to vacua with vanishing
cosmological constant.
