ON FIELD REDEFINITIONS AND EXACT-SOLUTIONS IN STRING THEORY

String backgrounds associated with gauged G/H WZNW models in general depend nontrivially on alpha'. We note, however that there exists a local covariant alpha'-dependent field redefinition that relates the exact metric-dilaton background corresponding to the SL(2, R)/U(1) model to its leading-order form (D = 2 black hole). As a consequence, there exists a ''scheme'' in which the string effective equations have the latter as an exact solution. However, the corresponding equation for the tachyon (which, like other Weyl anomaly equations, has scheme-dependent form) still contains corrections of all orders in alpha'. As a result, the string ''probes'' still feel the alpha'-corrected background. The field redefinitions we discuss contain the dilaton terms in the metric transformation law. We also comment on exact forms of the duality transformation in different ''schemes''.