  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
non-perturbative in $\alpha'$, thus suggesting that classical string theory may
realize de Sitter solutions in an unexpected fashion.
