  We elaborate a full superfield description of the interacting system of
dynamical D=4, N=1 supergravity and dynamical superstring. As far as minimal
formulation of the simple supergravity is used, such a system should contain as
well the tensor (real linear) multiplet which describes the dilaton and the
two-superform gauge field whose pull-back provides the Wess-Zumino term for the
superstring. The superfield action is given by the sum of the Wess-Zumino
action for D=4, N=1 superfield supergravity, the superfield action for the
tensor multiplet in curved superspace and the Green-Schwarz superstring action.
The latter includes the coupling to the tensor multiplet both in the Nambu-Goto
and in the Wess-Zumino terms. We derive superfield equations of motion
including, besides the superfield supergravity equations with the source, the
source-full superfield equations for the linear multiplet. The superstring
equations keep the same form as for the superstring in supergravity and
2-superform background. The analysis of gauge symmetries shows that the
superfield description of the interacting system is gauge equivalent to the
dynamical system described by the sum of the spacetime, component action for
supergravity interacting with tensor multiplet and of the purely bosonic string
action.
