  We present an algebraic approach to string theory, using a Hamiltonian
reduction of N=2 WZW models. An embedding of sl(1|2) in a Lie superalgebra
determines a niltopent subalgebra. Chirally gauging this subalgebra in the
corresponding WZW action leads to an extension of the N=2 superconformal
algebra. We classify all the embeddings of sl(1|2) into Lie superalgebras: this
provides an exhaustive classification and characterization of all extended N=2
superconformal algebras. Then, twisting these algebras, we obtain the BRST
structure of a string theory. We characterize and classify all the string
theories which can be obtained in this way.
  Based on a common work of E. Ragoucy, A. Sevrin and P. Sorba, presented by E.
Ragoucy at ``Extended and Quantum Algebras and their Applications to Physics'',
Nankai Institute in Tianjin (China) August 19-24 1996
