Cosmological solutions of type II superstrings

We study maximally symmetric cosmological solutions of type II supersymmetr ic strings in the presence of the exact quartic curvature corrections to th e lowest order effective action, including loop and D-instanton effects. We find that, unlike the case of type IIA theories, de Sitter solutions exist for type IIB superstrings, a conclusion that remains valid when higher-cur vature corrections are included on the basis of SL(2,Z) invariance. (C) 199 9 Elsevier Science B.V. Ail rights reserved.