Unorientable supersymmetric compactifications of M-theory

We discuss orbifold compactifications of the type II string, where the orbifold combines a free action on a base manifold and the $(-1{)}^{{\mathit{F}}_{\text{L}}}$ symmetry which acts by -1 on left-moving Ramond sectors. Such backgrounds can preserve minimal supersymmetry and lift to M-theory compactification on smooth unorientable manifolds.