Duality invariance: from M-theory to double field theory

We show how the duality invariant approach to M-theory formulated by Berman and Perry relates to the double field theory proposed by Hull and Zwiebach. In doing so we provide suggestions as to how Ramond fields can be incorporated into the double field theory. We find that the standard dimensional reduction procedure has a duality invariant (doubled) analogue in which the gauge fields of the doubled Kaluza-Klein ansatz encode the Ramond potentials. We identify the internal gauge index of these gauge fields with a spinorial index of O(d, d).