ß Symmetry of Supergravity.

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