Double Field Theory of Type II Strings
  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. 
Olaf Hohm, Seung Ki Kwak and Barton Zwiebach
hep-th