  The interplay between gravitational couplings on branes and the occurrence of
fractional flux in low dimensional orientifolds is examined. It 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 quantization 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 singularities.
