Abstract
String backgrounds associated with gauged $G/H$ WZNW models generically depend on $____alpha'$ or $1/k$. The exact expressions for the corresponding metric $G_{____m____n}$, antisymmetric tensor $B_{____m____n}$, and dilaton $____phi$ can be obtained by eliminating the $2d$ gauge field from the local part of the effective action of the gauged WZNW model. We show that there exists a manifestly gauge-invariant prescription for the derivation of the antisymmetric tensor coupling. When the subgroup $H$ is one-dimensional and $G$ is simple the antisymmetric tensor is given by the semiclassical ($____alpha'$-independent) expression. We consider in detail the simplest non-trivial example with non-trivial $B_{____m____n}$ -- the D=3 sigma model corresponding to the $[SL(2,R) x R]/R$ gauged WZNW theory (`charged black string') and show that the exact expressions for $G_{____m____n}$, $B_{____m____n}$ and $____phi$ solve the Weyl invariance conditions in the two-loop approximation. Similar conclusion is reached for the closely related $SL(2,R)/R$ chiral gauged WZNW model. We find that there exists a scheme in which the semiclassical background is also a solution of the two-loop conformal invariance equations (but the tachyon equation takes a non-canonical form). We discuss in detail the role of field redefinitions (scheme dependence) in establishing a correspondence between the sigma model and conformal field theory results.