Yukawa Couplings for Bosonic $Z_N$ Orbifolds: Their Moduli and Twisted   Sector Dependence
  The three point correlation functions with twist fields are determined for bosonic $Z_N$ orbifolds. Both the choice of the modular background (compatible with the twist) and of the (higher) twisted sectors involved are fully general. We point out a necessary restriction on the set of instantons contributing to twist field correlation functions not obtained in previous calculations. Our results show that the theory is target space duality invariant. 
S. Stieberger, D. Jungnickel, J. Lauer, M. Spalinski