EXACT 4-DIMENSIONAL STRING SOLUTIONS AND TODA-LIKE SIGMA-MODELS FROM NULL-GAUGED WZNW THEORIES

We construct a new class of exact string solutions with a four-dimensi onal target spent metric of signature (-, +, +, +) by gauging the inde pendent left and right nilpotent subgroups with 'null' generators of W ZNW models for rank-2 non-compact groups null' property of the generat ors (Tr(N(n)N(m) = 0) implies the consistency of the gauging and the a bsence of alpha'-corrections to the semiclassical backgrounds obtained from the gauged WZNW models. In the case of the maximally non-compact groups (G = SL(3), SO(2, 2), SO(2, 3), G2) the construction correspon ds to gauging some of the subgroups generated by the nilpotent 'step' operators in the Gauss decomposition. The rank-2 case is a particular example of a general construction leading to conformal backgrounds wit h one time-like direction. The conformal theories obtained by integrat ing out the gauge field can be considered as sigma model analogs of To da models (their classical equations of motion are equivalent to Toda model equations). The procedure of 'null gauging' applies also to othe r non-compact groups. As an example, we consider the gauging of SO(1, 3) where the resulting metric has the signature (-, -, +, +) but admit s two analytic continuations with Minkowski signature. The backgrounds we find have '2 + 2' structure with two null Killing vectors. Their d ual counterparts have one covariantly constant null Killing vector, i. e. are of 'plane-wave' type (with metric and dilaton depending only on transverse spatial coordinates) and also represent exact string solut ions.