A Lorentz invariant doubled world-sheet theory

We propose a Lorentz invariant version of Tseytlin's doubled world-sheet theory that makes T-duality covariance of the string manifest. This theory can be derived as a gauge fixed version of Buscher's gauging procedure, in which the leftover gauge field component acts as a Lagrange multiplier. This description can naturally account for fractional linear $O(D,D)$ transformations of the metric and $b$ field. It is capable of describing nongeometric backgrounds; geometric and nongeometric fluxes are encoded in the doubled antisymmetric tensor field strength.