U DUALITY AND SYMPLECTIC FORMULATION OF DILATON-AXION GRAVITY

We study a bosonic four-dimensional effective action corresponding to the heterotic string compactified on a six-tonus (dilaton-axion gravit y with one vector field) on a curved space-time manifold possessing a timelike Killing vector field. Previously the existence of the SO(2,3) similar to Sp(4,R) global symmetry (U duality) as well as the symmetri c space property of the corresponding sigma model have been establishe d following the Neugebauer-Kramer approach. Here we present an explici t form of the Sp(4,R) generators in terms of coset variables and const ruct a representation of the coset in terms of the physical target spa ce coordinates. A complex symmetric 2X2 matrix Z (''matrix dilaton-axi on'') is then introduced for which U duality takes the matrix-valued S L (2,R) form. In terms of this matrix the theory is further presented as a Kahler sigma model. This leads to a more economic 2X2 formulation suggesting some new solution-generating techniques. A new solution (c orresponding to constant ImZ) is obtained which describes the system o f point massless magnetic monopoles endowed with axion charges. In suc h a system mutual magnetic repulsion is exactly balanced by axion attr action so that the resulting space-time is focally flat but possesses multiple Taub-NUT singularities.