Type IIA Orientifolds on General Supersymmetric Z_N Orbifolds
  We construct Type IIA orientifolds for general supersymmetric Z_N orbifolds. In particular, we provide the methods to deal with the non-factorisable six-dimensional tori for the cases Z7, Z8, Z8', Z12 and Z12'. As an application of these methods we explicitly construct many new orientifold models. 
Ralph Blumenhagen, Joseph P. Conlon, Kerim Suruliz