D=4, N=1 orientifolds with vector structure

We construct compact type IIB orientifolds with discrete groups Z(4), Z(6), Z'(6), Z(8), Z(12) and Z'(12). These models are N = 1 supersymmetric in D = 4 and have vector structure. The possibility of having vector structure i n Z(N) orientifolds with even N arises due to an alternative Omega -project ion in the twisted sectors. Some of the models without vector structure are known to be inconsistent because of uncancelled tadpoles. We show that vec tor structure leads to a sign flip in the twisted Klein bottle contribution . As a consequence, all the tadpoles can be cancelled by introducing D9-bra nes and D5-branes. (C) 2001 Elsevier Science B.V. All rights reserved.