GRAVITATIONAL COUPLINGS AND Z(2) ORIENTIFOLDS

The interplay between gravitational couplings on branes and the occurr ence of fractional flux in low-dimensional orientifolds is examined, I t is argued that gravitational couplings need to be assigned not only to D-branes but also to orientifold planes, The fractional charges of the orientifold d-planes can be understood in terms of flux quantizati on of the d-3 form potential and modified Bianchi identities, Detailed results are presented for the case of the type IIB orientifold on T-6 /Z(2), which is dual to F-theory on a complex 4-fold with terminal sin gularities, (C) 1998 Elsevier Science B.V.