DUALITY INVARIANT CLASS OF EXACT STRING BACKGROUNDS

We consider a class of (2+D)-dimensional string backgrounds with a tar get space metric having a covariantly constant null Killing vector and flat ''transverse'' part. The corresponding sigma models are invarian t under D abelian isometries and are transformed by O(D, D) duality in to models belonging to the same class. The leading-order solutions of the conformal invariance equations (metric, antisymmetric tensor and d ilaton), as well as the action of O(D, D) duality transformations on t hem, are exact, i.e. are not modified by alpha'-corrections. This make s a discussion of different space-time representations of the same str ing solution (related by the O(D, D\Z) duality subgroup) rather explic it. We show that the O(D, D) duality may connect curved (2+D)-dimensio nal backgrounds with solutions having flat metric but, in general, non -trivial antisymmetric tensor and dilation. We discuss several particu lar examples including the (2+D=4)-dimensional background that was rec ently interpreted in terms of a WZW model.