1. For reviews, see J. Bagger, Lectures at the 1988 Banff Summer Institute, preprint HUTP89/A006;
2. Lectures at Les Houches Summer School 1988;Ginsparg,1989
3. F. Ravinini, preprint NORDITA-89/21P
4. Infinite dimensional Lie algebras;Kac,1985
5. Heterotic string theory (I). The free heterotic string