Green-Schwarz Mechanism for String Dualities

We determine the complete spacetime action to first order in α^{'} for the massless fields of bosonic string theory compactified on a d-dimensional torus. A fully systematic procedure is developed that brings the action into a minimal form in which all fields apart from the metric enter only with first-order derivatives. T duality implies that this action must have a global O(d,d,R) symmetry, and we show that this requires a Green-Schwarz type mechanism for α^{'}-deformed O(d,d,R) transformations. In terms of a frame formalism with GL(d)×GL(d) gauge symmetry this amounts to a modification of the three-form curvature by a Chern-Simons term for composite gauge fields.