  We report two surprising results on $\alpha'$ corrections in string theory
restricted to massless fields. First, for critical dimension Bianchi type I
cosmologies with $q$ scale factors only $q-1$ of them have non-trivial
$\alpha'$ corrections. In particular, for FRW backgrounds all $\alpha'$
corrections are trivial. Second, in non-critical dimensions, all terms in the
spacetime action other than the cosmological term are field redefinition
equivalent to terms with arbitrarily many derivatives, with the latter
generally of the same order. Assuming an $\alpha'$ expansion with coefficients
that fall off sufficiently fast, we consider field redefinitions consistent
with this fall-off and classify the higher derivative terms for two-dimensional
string theory with one timelike isometry. This most general duality-invariant
theory permits black-hole solutions, and we provide perturbative and
non-perturbative tools to explore them.
