I discuss the higher-derivative \alpha' corrections in the reduction to one dimension (cosmic time) that is relevant for the dynamics of cosmological backgrounds. In this setting string theory is known to be duality invariant under O(d,d), which makes it possible to completely classify all \alpha' corrections. I discuss some generic features of the \alpha' completed Friedmann equations and show that there are duality invariant theories that allow for de Sitter solutions that are non-perturbative in \alpha', suggesting that string theory may permit de Sitter vacua in this novel fashion.