  We investigate the connections between four-dimensional, N=2 M-theory vacua
constructed as orbifolds of type II, heterotic, and type I strings. All these
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 evidence 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 particular we
consider gravitational corrections to the effective action.
