Non-perturbative string connections

We discuss the duality between two type I compactifications to four dimensions and an heterotic construction with spontaneous breaking of the N=4 supersymmetry to N=2. This duality allows us to gain insight into the non-perturbative properties of these models. Through the analysis of the gravitational corrections, we then investigate the connections between four-dimensional, N=2 M-theory vacua constructed as orbifolds of type II, heterotic, and type I strings.Comment: 7 pages, latex, to appear in the proceedings of the TMR meeting on Quantum aspects of gauge theories, supersymmetry and unification, Paris, 1--7 September 199

Type IIB orbifolds with D5-branes and their string duals

We consider Z2, freely acting orbifolds of the type IIB string with 16 parallel D5-branes. When the string is compactified on T2 X T4 and the D5-branes are wrapped on T2, these systems possess N=2 supersymmetry, originating from the spontaneous, partial breaking of N=4. Extended supersymmetry allows us to investigate the duality with certain heterotic and type I constructions, and to obtain informations about their non-perturbative regime.Comment: latex, 20 pages, no figures, revised version to be published on Nuclear Physics