Cosmological solutions with dilaton and maximally symmetric space in   string theory
  We discuss time - dependent solutions of the leading order string effective equations for a non-zero central charge deficit and curved maximally symmetric space. Some regular solutions are found for the case of non-trivial antisymmetric tensor and vector backgrounds (in various dimensions) and negative spatial curvature. It remains an open question which conformal theories are exact generalisations of these solutions. 
A.A. Tseytlin