  We consider the target space theory of bosonic and heterotic string theory to
first order in $\alpha'$ compactified to three dimensions, using a formulation
that is manifestly T-duality invariant under ${\rm O}(d,d,\mathbb{R})$ with
$d=23$ and $d=7$, respectively. While the two-derivative supergravity exhibits
a symmetry enhancement to the U-duality group ${\rm O}(d+1,d+1)$, the
continuous group is known to be broken to ${\rm O}(d,d,\mathbb{R})$ by the
first $\alpha'$ correction. We revisit this observation by computing the full
effective actions in three dimensions to first order in $\alpha'$ by dualizing
the vector gauge fields. We give a formally ${\rm O}(d+1,d+1)$ invariant
formulation by invoking a vector compensator, and we observe a chiral pattern
that allows one to reconstruct the bosonic action from the heterotic action.
Furthermore, we obtain a particular massive deformation by integrating out the
external $B$ field. This induces a novel Chern-Simons term based on composite
connections that, remarkably, is ${\rm O}(d+1,d+1)$ invariant to leading order
in the deformation parameter.
