Constraints on string cosmology

String theory contains sources like orientifold planes that support higher derivative interactions. These interactions make static flux compactifications possible which are forbidden in supergravity. They can also lead to violations of the strong energy condition which is needed for an accelerating universe. We examine how large a violation is possible in the context of the heterotic string compactified to four dimensions. We find that de Sitter solutions are still not possible but that classically forbidden anti-de Sitter solutions are possible.