  Continuous O$(d,d)$ global symmetries emerge in Kaluza-Klein reductions of
$D$-dimensional string supergravities to $D-d$ dimensions. We show that the
non-geometric elements of this group effectively act in the $D$-dimensional
parent theory as a hidden bosonic symmetry that fixes its couplings: the
$\beta$-symmetry. We give the explicit $\beta$-transformations to first order
in $\alpha'$ and verify the invariance of the action as well as the closure of
the transformation rules.
