Non-perturbative de Sitter vacua via $\alpha'$ corrections

The higher-derivative α′ corrections consistent with O(d,d) duality invariance can be completely classified for cosmological, purely time-dependent backgrounds. This result is used to show that there are duality invariant theories featuring string-frame de Sitter vacua as solutions that are nonperturbative in α′, thus suggesting that classical string theory may realize de Sitter solutions in an unexpected fashion.