  Einstein's theory in the vacuum was recently shown to possess an $SO(2)$
duality invariance, which is broken by coupling to matter. Duality invariance
can be restored by enlarging the phase space of the theory to allow for
violations of the algebraic Bianchi identity. We show that in cases where the
matter content can be understood as a component of the torsion tensor duality
can be restored and we compute the corresponding duality current. We consider
the case of NS-NS gravity as an example and find that the duality current is
given by the divergence of the axion. In the linearized approximation of the
low energy heterotic string theory these results imply that duality of the
generalized Riemann curvature tensor implements Riemannian, axion-dilaton and
electro-magnetic dualities simultaneously.
