Superstrings with spontaneously broken supersymmetry and their effective theories

We discuss superstring models with spontaneous breaking of N=1, 2 or 4 space-time supersymmetry, via coordinate-dependent compactifications from five to four dimensions. We provide a description of the supersymmetry breaking mechanism in the formulation with complex world-sheet fermions, and we reinterpret it in terms of special deformations of the lorentzian charge lattice. Considering a representative string model with spontaneously broken N=1 supersymmetry and massless chiral fermions, we show that the knowledge of the spectrum of states and of the flat directions completely specifies its low-energy effective lagrangian, which turns out to be a new no-scale supergravity model. We outline the qualitative difference between these models and other scenarios of gaugino (gravitino) condensation or general non-perturbative phenomena. © 1989.