Four-dimensional N=2 superstring constructions and their (non-) perturbative duality connections

We investigate the connections between four-dimensional, N=2 M-theory vacua constructed as orbifolds of type TI, heterotic, and type I strings. All th ese models have the same massless spectrum, which contains an equal number of vector multiplets and hypermultiplets, with a gauge group of the maximal rank allowed in a perturbative heterotic string construction. We find evid ence for duality between two type I compactifications recently proposed and a new heterotic construction that we present here. This duality allows us to gain insight into the non-perturbative properties of these models, In pa rticular we consider gravitational corrections to the effective action. (C) 1999 Elsevier Science B,V. All rights reserved.