  We derive fully covariant expressions for disk scattering amplitudes of any
two massless closed strings in which mixed Neumann and Dirichlet world-sheet
boundary conditions are included. From the two-point amplitudes, we derive the
long range background fields and verify that they correspond to D$p$-brane
bound state. Also, from the scattering amplitudes, we calculate the linear
coupling of closed string fields to D-brane world-volume and show that they are
consistent with Born-Infeld and Chern-Simons actions in the presence of a
background field.
