2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/36443We study maximally symmetric cosmological solutions of type II supersymmetric strings in the presence of the exact, SL(2,Z)-invariant, higher-curvature corrections to the lowest order effective action. We find that, unlike the case of type IIA theories, de Sitter solutions exist, at all orders in $α'$, for type IIB superstrings, when non-perturbative instanton-effects are included on the basis of SL(2,Z) invariance.5 pages, Latex, no figures, talk presented at "New Worlds in Astroparticle Physics" 1998, Faro, PortugalGeneral Relativity and Quantum Cosmologytext