EXACT-SOLUTIONS OF CLOSED STRING THEORY

We review explicitly known exact D = 4 solutions with Minkowski signat ure in closed bosonic string theory. Classical string solutions with s pacetime interpretation are represented by conformal sigma models. Two large (intersecting) classes of solutions are described by gauged WZW models and 'chiral null models' (models with conserved chiral null cu rrent). The latter class includes plane-wave-type backgrounds (admitti ng a covariantly constant null Killing vector) and backgrounds with tw o null Killing vectors (e.g. fundamental string solution). D > 4 chira l null models describe some exact D = 4 solutions with electromagnetic fields, for example, extreme electric black holes, charged fundamenta l strings and their generalizations. In addition, there exists a class of conformal models representing axially symmetric stationary magneti c flux tube backgrounds (including, in particular, the dilatonic Melvi n solution). In contrast to spherically symmetric chiral null models f or which the corresponding conformal field theory is not known explici tly, the magnetic Bur tube models (together with some non-semisimple W ZW models) are among the first examples of solvable unitary conformal string models with non-trivial D = 4 curved spacetime interpretation. For these models one is able to express the quantum Hamiltonian in ter ms of free fields and to find explicitly the physical spectrum and str ing partition function.