Heterotic effective action and duality symmetries revisited

The dimensional reduction of heterotic supergravity with gauge fields truncated to the Cartan subalgebra exhibits a continuous O (d, d + 16; R) global symmetry, related to the O (d, d + 16; Z) T-duality of heterotic strings on a d-torus. The O (d, d + 16; R) symmetry is not present, however, if the supergravity reduction is done including the full set of E-8 x E-8 or SO (32) gauge fields. We analyze which duality symmetries are realized to all orders in alpha' in the proper effective field theories for the massless string states. We find a universal O (d, d; R) symmetry, also predicted by double field theory. We confirm this by giving a novel formulation of the dimensionally reduced supergravity in terms of O (d, d) multiplets, and we discuss cases of symmetry enhancement.