  Integrable structure of the symmetry reduced dynamics of massless bosonic
sector of the heterotic string effective action is presented. For string
background equations that govern in the space-time of $D$ dimensions ($D\ge 4$)
the dynamics of interacting gravitational, dilaton, antisymmetric tensor and
any number $n\ge 0$ of Abelian vector gauge fields, all depending only on two
coordinates, we construct an \emph{equivalent} $(2 d+n)\times(2 d+n)$ matrix
spectral problem ($d=D-2$). This spectral problem provides the base for the
development of various solution constructing procedures (dressing
transformations, integral equation methods). For the case of the absence of
Abelian gauge fields, we present the soliton generating transformations of any
background with interacting gravitational, dilaton and the second rank
antisymmetric tensor fields. This new soliton generating procedure is available
for constructing of various types of field configurations including stationary
axisymmetric fields, interacting plane, cylindrical or some other types of
waves and cosmological solutions.
