PLANE GRAVITATIONAL-WAVES IN STRING THEORY

We analyze the coset model (E2c x E2c)/E2c and construct a class of ex act string vacua which describe plane gravitational waves and their du als, generalizing the plane wave background found by Nappi and Witten. In particular, the vector gauging describes a two-parameter family of singular geometries with two isometries, which is dual to plane gravi tational waves. In addition, there is a mixed vector-axial gauging whi ch describes a one-parameter family of plane waves with five isometrie s. These two backgrounds are related by a duality transformation which generalizes the known axial-vector duality for abelian subgroups.