1. Candelas, P., Horowitz, G., Strominger, A., Witten, E.: Nucl. Phys.B 258, 46 (1985)
2. Dixon, L.: Some world-sheet properties of superstring compactifications, on orbifolds and otherwise. Princeton University preprint, PUTP-1074
3. See, for example, Dijkgraaf, R., Verlinde, E., Verlinde, H.: Commun. Math. Phys.115, 649 (1988) and references therein
4. Friedan, D., Kent, A., Shenker, S., Witten, E.: In preparation
5. Gepner, D.: Phys. Lett.199B, 380 (1987)