Summary: We use double field theory to give a unified description of the low energy limits of type IIA and type IIB superstrings. The Ramond-Ramond potentials fit into spinor representations of the duality group \(O(D, D)\) and field-strengths are obtained by acting with the Dirac operator on the potentials. The action, supplemented by a Spin\({}^{+}(D, D)\)-covariant self-duality condition on field strengths, reduces to the IIA and IIB theories in different frames. As usual, the NS-NS gravitational variables are described through the generalized metric. Our work suggests that the fundamental gravitational variable is a hermitian element of the group Spin(\(D\), \(D\)) whose natural projection to \(O(D, D)\) gives the generalized metric.