  A supersymmetric action functional describing the interaction of the
fundamental superstring with the D=10, type IIB Dirichlet super-9-brane is
presented. A set of supersymmetric equations for the coupled system is obtained
from the action principle. It is found that the interaction of the string
endpoints with the super-D9-brane gauge field requires some restrictions for
the image of the gauge field strength. When those restrictions are not imposed,
the equations imply the absence of the endpoints, and the equations coincide
either with the ones of the free super-D9-brane or with the ones for the free
closed type IIB superstring. Different phases of the coupled system are
described. A generalization to an arbitrary system of intersecting branes is
discussed.
