Non-commutativity and non-associativity of the doubled string in non-geometric backgrounds

A bstract We use a T-duality invariant action to investigate the behaviour of a string in non-geometric backgrounds, where there is a non-trivial global O ( D, D ) patching or monodromy. This action leads to a set of Dirac brackets describing the dynamics of the doubled string, with these brackets determined only by the monodromy. This allows for a simple derivation of non-commutativity and non-associativity in backgrounds which are (even locally) non-geometric. We focus here on the example of the three-torus with H-flux, finding non-commutativity but not non-associativity. We also comment on the relation to the exotic 5 2 2 brane, which shares the same monodromy.