On the existence and stability of maximally symmetric cosmological solutions in superstring theories

We study the existence and stability of maximally symmetric cosmological so lutions in bosonic, heterotic and supersymmetric string theories in the eff ective Lagrangian approach. We find that, at tree level, de Sitter and anti -de Sitter solutions are excluded for all theories and only Minkowsky space time remains a (stable) solution. For type IIB superstrings, however, such solutions exist if the exact higher-curvature corrections to the lowest ord er effective action, which contain loop as well as non-perturbative D-insta nton corrections, are included on the basis of SL(2,Z) invariance.