Type IIB orbifolds with D5-branes and their string duals

We consider Z(2), freely acting orbifolds of the type IIB string with 16 pa rallel DS-branes. When the string is compactified on T-2 x T-4 and the D5-b ranes are wrapped on T-2, these systems possess N-4 = 2 supersymmetry, orig inating from the spontaneous, partial breaking of N-4 = 4. Extended supersy mmetry allows us to investigate the duality with certain heterotic and type I constructions, and to obtain information about their non-perturbative re gime. (C) 2000 Elsevier Science B.V. All rights reserved.