On-mass-shell gaugino condensation in ZN orbifold compactifications

We discuss non-perturbative aspects of string effective field theories with N = 1 supersymmetry in four dimensions. By the use of a scalar potential, which is on-shell invariant under the supersymmetric duality of the dilaton, we study gaugino condensation in (2,2) symmetric ZN orbifold compactifications. The duality under consideration relates a two-form antisymmetric tensor to a pseudoscalar. We show that our approach is independent of the superfield-representation of the dilaton and preserves the U(1)PQ Peccei-Quinn symmetry exactly.