An O(d,d) symmetry of the manifold of string vacua that do not depend on
d (out of D) space-time coordinates has been recently identified. Here we
write down, for d=D−1, the low energy equations of motion and their general
solution in a manifestly O(d,d)-invariant form, pointing out an amusing
similarity with the renormalization group framework. Previously considered
cosmological and black hole solutions are recovered as particular examples.Comment: 9 page