Stationary BPS solutions to dilaton-axion gravity

Stationary four-dimensional BPS solutions to gravity-coupled bosonic theories admitting a three-dimensional sigma-model representation on coset spaces are interpreted as null geodesics of the target manifold equipped with a certain number of harmonic maps. For asymptotically flat (or Taub-NUT) space-times such geodesics can be directly parametrized in terms of charges saturating the Bogomol'nyi-Gibbons-Hull bound, and classified according to the structure of related coset matrices. We investigate in detail the ''dilaton-axion gravity'' with one vector field, and show that in the space of BPS solutions an SO(1,2)XSO(2) classical symmetry is acting. Within the present formalism the most general multicenter (IWP or Taub-NUT dyon) solutions are derived in a simple way. We also discover a large new class of asymptotically flat solutions for which the dilaton and axion charges are constrained only by the BPS bound. The string metrics for these solutions are generically regular. Both the IWP class and the new class contain massless solutions.