Flux compactifications, twisted tori and doubled geometry

In arXiv:0902.4032 [hep-th] an O(D,D)-covariant sigma model describing the embedding of a closed world-sheet into the 2D-dimensional twisted torus was proposed. Such sigma models provide a universal description of string theory with target spaces related by the action of T-duality. In this article a six-dimensional toy example is studied in detail. Different polarisations of the six-dimensional target space give different three-dimensional string backgrounds including a nilmanifold with H-flux, a T-fold with R-flux and a new class of T-folds. Global issues and connections with the doubled torus formalism are discussed. Finally, the sigma model introduced in arXiv:0902.4032 [hep-th], describing the embedding of a world-sheet into the doubled twisted torus, is generalised to one describing a target space which is a bundle of the doubled twisted torus over a base, allowing for a more complete description of the associated gauged supergravity from the world-sheet perspective to be given.