  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.
