Supersymmetric Z_N times Z_M Orientifolds in 4D with D-Branes at Angles
  We construct orientifolds of type IIA string theory. The theory is compactified on a T^6/Z_N times Z_M orbifold. In addition worldsheet parity in combination with a reflection of three compact directions is modded out. Tadpole cancellation requires to add D-6-branes at angles. The resulting four dimensional theories are N=1 supersymmetric and non-chiral. 
Stefan Forste, Gabriele Honecker and Ralph Schreyer