GENERALIZED CHIRAL NULL MODELS AND ROTATING STRING BACKGROUNDS

We consider an extension of a special class of conformal sigma models ('chiral null models') which describe extreme supersymmetric string so lutions. The new models contain both 'left' and 'right' vector couplin gs and should correspond to non-BPS backgrounds. In particular, we dis cuss a conformal six-dimensional model which is a combination of funda mental string and 5-brane models with the two extra couplings represen ting rotations in the orthogonal planes. If the two rotation parameter s are independent the resulting background is found to be either singu lar or non-asymptotically flat. The non-asymptotically flat solution h as a regular short distance limit described by a 'twisted' product of SL(2,R) and SU(2) WZW theories with two twist pararmeters mixing the i sometric Euler angles of SU(2) with a null direction of SL(2,R).