MATRIX DILATON-AXION FOR THE HETEROTIC STRING IN 3 DIMENSIONS

A new and surprisingly simple representation is found for the heteroti c string bosonic effective action in three dimensions in terms of comp lex potentials. The system is presented as a Kahler sigma-model using a complex symmetric 2 x 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 t en isometries of the target space (U-duality transformations) are foun d and used to generate the non-trivial Ehlers-Harrison sector from the gauge one. Finite transformations are obtained in a simple form in te rms of complex potentials. The new solution generating technique is us ed to construct the EMDA double-Kerr solution.