STRINGS FROM N=2 GAUGED WESS-ZUMINO-WITTEN MODELS

We present an algebraic approach to string theory. An embedding of sl( 2/1) in a super Lie algebra together with a grading on the Lie algebra determines a nilpotent subalgebra of the super Lie algebra. Chirally gauging this subalgebra in the corresponding Wess-Zumino-Witten model, breaks the affine symmetry of the Wess-Zumino-Witten model to some ex tension of the N = 2 superconformal algebra. The extension is complete ly determined by the sl(2/1) embedding, The realization of the superco nformal algebra is determined by the grading. For a particular choice of grading, one obtains in this way, after twisting, the BRST structur e of a string theory. We classify all embeddings of sl(2/1) into Lie s uper algebras and give a detailed account of the branching of the adjo int representation. This provides an exhaustive classification and cha racterization of both all extended N = 2 superconformal algebras and a ll string theories which can be obtained in this way.