Open descendants of Z2 x Z2 freely acting orbifolds

We discuss Z(2) X Z(2) orientifolds where the orbifold twists are accompanied by shifts on momentum or winding lattice states, The models contain variable numbers of D5-branes, whose massless land, at times, even massive) modes have variable numbers of supersymmetries, We display new type-I models with partial supersymmetry breaking N = 2 --> N = 1, N = 4 --> N = 1 and N = 4 --> N = 2. The geometry of these models is rather rich: the shift operations create brane multiplets related by orbifold transformations that support gauge groups of reduced rank. Some of the models are deformations of six-dimensional supersymmetric type-I models, while others have dual M-theory descriptions. (C) 2000 Elsevier Science B.V. All rights reserved.