Unification of Type-II Strings and T Duality

We present a unified description of the low-energy limits of type-II string theories. This is achieved by a formulation that doubles the space-time coordinates in order to realize the T-duality group O(10; 10) geometrically. The Ramond-Ramond fields are described by a spinor of O(10; 10), which couples to the gravitational fields via the Spind(10; 10) representative of the so-called generalized metric. This theory, which is supplemented by a T-duality covariant self-duality constraint, unifies the type-II theories in that each of them is obtained for a particular subspace of the doubled space.