M theory, orientifolds and G-flux

We study the properties of M and F theory compactifications to three and fo ur dimensions with background fluxes. We provide a simple construction of s upersymmetric vacua, including some with orientifold descriptions. These va cua, which have warp factors, typically have fewer moduli than conventional Calabi-Yau compactifications. The mechanism for anomaly cancellation in th e orientifold models involves background RR and NS fluxes. We consider in d etail an orientifold of K3 x T-2 with background fluxes. After a combinatio n of T and S-dualities, this type IIB orientifold is mapped to a compactifi cation of the SO(32) heterotic string on a non-Kahler space with torsion.