  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.
