Generalised 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 solutions. The new models contain both `left' and `right' vector couplings and should correspond to non-BPS backgrounds. In particular, we discuss a conformal six-dimensional model which is a combination of fundamental string and 5-brane models with two extra couplings representing rotations in orthogonal planes. If the two rotational parameters are independent the resulting background is found to be either singular or not asymptotically flat. The non asymptotically flat solution has a regular short distance limit described by a `twisted' product of SL(2,R) and SU(2) WZW theories with two twist parameters mixing the isometric Euler angles of SU(2) with a null direction of SL(2,R).