Extremal rotating dipole solution in four-dimensional heterotic string theory

We develop new Lagrange formulation of the four-dimensional low-energy heterotic string theory in the stationary case. Using this formulation we derive three non-extremal and one extremal two-dimensional subspaces invariant under the action of all dualities which preserve spatial field asymptotics. For the extremal subspace we construct the solution which describes a massless rotating dipole source.