2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/198720We 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.latex, 20 pages, no figures, revised version to be published on Nuclear Physics BHigh Energy Physics - Theorytext