Maximally symmetric cosmological solutions of higher-curvature string effective theories with dilatons

We study the effect of higher-curvature terms in the string low-energy effective actions on the maximally symmetric cosmological solutions of the theory, up to corrections quartic in the curvatures, for the bosonic and heterotic strings as well as for the type II superstring. We find that such solutions exist for all string types but that they always disappear when the dilaton field is included, a conclusion that can be avoided if string-loop effects are taken into account.