Orbifolds of M-theory

We study Z(2)-orbifolds of 11-dimensional M-theory on tori of various dimensions. The most interesting model (besides the known S-1/Z(2) case) corresponds to T-5/Z(2), for which we argue that the resulting six-dimensional theory is equivalent to the type IIB string compactified on K3. Gravitational anomaly cancellation plays a crucial role in determining what states appear in the twisted sector, Most of the other models appear to break spacetime supersymmetry. We observe that M-theory tends to produce chiral compactifications on orbifolds, and that our results may provide an insight into the mechanism by which twisted-sector states arise in this hypothetical theory.