1. Green, M. B., Schwarz, J. H., Witten, E.: Superstring theory Vols I, II, Cambridge. Cambridge University Press 1987
2. For a review see D'Hoker, E., Phong, D. H.: Rev. Mod. Phys.60, 917 (1988)
3. Fradkin, E.S., Tseytlin, A.A.: Nucl. Phys.B261, 1 (1985) Maharana, J., Veneziano, G.: Nucl. Phys.B 283, 126 (1987)
4. There is a vast literature on this subject. An up-to-date reference can be found in review articles: Sen, A.: In: Superstrings, Unified Theories, and Cosmology (Proc. of the Summer Workshop in High Energy Physics and Cosmology, Trieste, 1986), Furlan, G., Jengo, R., Pati, J.C., Sciama, D.W., Shafi, Q. (eds.). Singapore: World Scientific Callan, C.G., Thorlacius, L.: SLAC Preprint 1989 Tseytlin, A.A.: Int. J. Mod. Phys. A4, 1257 (1989)
5. Proc. of the Trieste Spring School;L. Alvarez-Gaumé,1987