Duality Invariance and Higher Derivatives

We dimensionally reduce the spacetime action of bosonic string theory, and that of the bosonic sector of heterotic string theory after truncating the Yang-Mills gauge fields, on a $d$-dimensional torus including all higher-derivative corrections to first order in $\alpha'$. A systematic procedure is developed that brings this action into a minimal form in which all fields except the metric carry only first order derivatives. This action is shown to be invariant under ${\rm O}(d,d,\mathbb{R})$ transformations that acquire $\alpha'$-corrections through a Green-Schwarz type mechanism. We prove that, up to a global pre-factor, the first order $\alpha'$-corrections are uniquely determined by ${\rm O}(d,d,\mathbb{R})$ invariance.