Open Descendants of Freely Acting Z2 x Z2 Orientifolds (TOPCITE: 65 citazioni su SPIRES HEP)

We discuss Z2=Z2 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 and, at times, even massive. modes have variable numbers of supersymmetries. We display new type-I models with partial supersymmetry breaking Ns2™Ns1, Ns4™Ns1 and Ns4™Ns2. 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.