  A bosonic sector of the four-dimensional low-energy heterotic string theory
with two Abelian gauge fields is considered in the stationary case. A new 4X4
unitary null-curvature matrix representation of the theory is derived and the
corresponding formulation based on the use of a new 2X2 Ernst type matrix
complex potential is developed. The group of hidden symmetries is described and
classified in the matrix-valued quasi General Relativity form. A subgroup of
charging symmetries is constructed and representation which transforms linearly
under the action of this symmetry subgroup is established. Also the solution
generation procedure based on the application of the total charging symmetry
subgroup to the stationary Einstein-Maxwell theory is analyzed.
