Sigma-model anomalies in compact D=4, N=1 Type IIB orientifolds and Fayet-Iliopoulos terms

Compact Type IIB D = 4, N = 1 orientifolds have certain U(1) sigma-model sy mmetries at the level of the effective Lagrangian. These symmetries are gen erically anomalous. We study the particular case of Z(N) orientifolds and f ind that these anomalies may be cancelled by a generalized Green-Schwarz me chanism. This mechanism works by the exchange of twisted RR-fields associat ed to the orbifold singularities and it requires the mixing between twisted and untwisted moduli of the orbifold. As a consequence, the Fayet-Iliopoul os terms which are present for the gauged anomalous LT(1)'s of the models g et an additional untwisted modulus dependent piece at the tree level. (C) 2 000 Elsevier Science B.V. All rights reserved.