Nonperturbative de Sitter vacua via α′ corrections

The higher-derivative alpha' 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 alpha', thus suggesting that classical string theory may realize de Sitter solutions in an unexpected fashion.