Exact string solutions and duality
  We review known exact classical solutions in (bosonic) string theory. The main classes of solutions are `cosets' (gauged WZW models), `plane wave'-type backgrounds (admitting a covariantly constant null Killing vector) and `$F$-models' (backgrounds with two null Killing vectors generalising the `fundamental string' solution). The recently constructed $D=4$ solutions with Minkowski signature are given explicitly. We consider various relations between these solutions and, in particular, discuss some aspects of the duality symmetry. [To appear in the Proceedings of the 2nd Journe'e Cosmologie, Observatoire de Paris, June 2-4, 1994.] 
A.A. Tseytlin