Duality and Orbifolds
  We construct several examples where duality transformation commutes with the orbifolding procedure even when the orbifolding group does not act freely, and there are massless states from the twisted sector at a generic point in the moduli space. Often the matching of spectrum in the dual theories is a result of non-trivial identities satisfied by the coefficients of one loop tadpoles in the heterotic, type II and type I string theories. 
Ashoke Sen