Discrete deformations in Type I vacua

We study supersymmetric orientifolds where the world-sheet parity transform ation is combined with a conjugation of some compact complex coordinates. W e investigate their T-duality relation to standard orientifolds and discuss the origin of continuous and discrete moduli. In contrast to standard orie ntifolds, the antisymmetric tensor describes a continuous deformation, whil e the off-diagonal, part of the metric is frozen to quantized values and is responsible for the rank reduction of the gauge group. We also give a geom etrical interpretation of some recently constructed six-dimensional permuta tional orientifolds. (C) 2000 Published by Elsevier Science B.V. All rights reserved.