  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.
