Matrix dilaton-axion for the heterotic string in three dimensions

A new and surprisingly simple representation is found for the heterotic string bosonic effective action in three dimensions in terms of complex potentials. The system is presented as a Kähler σ-model using a complex symmetric 2 × 2 matrix (matrix dilaton-axion) which depends linearly on three Ernst-type potentials and transforms under U-duality via the matrix valued SL(2, R). Two discrete automorphisms relating ten isometries of the target space (U-duality transformations) are found and used to generate the non-trivial Ehlers-Harrison sector from the gauge one. Finite transformations are obtained in a simple form in terms of complex potentials. The new solution generating technique is used to construct the EMDA double-Kerr solution.