String Dualities at Order α^{'3}.

We compute the cosmological reduction of the fourth powers of the Riemann tensor claimed to arise in string theory at order α^{'}^{3}, with an overall coefficient proportional to ζ(3), and show that it is compatible with an O(9,9) symmetry. This confirms the general result in string theory, due to Sen [O(d)×O(d) symmetry of the space of cosmological solutions in string theory, scale factor duality and two-dimensional black holes, Phys. Lett.