Superstring scattering from D-branes

We derive fully covariant expressions for all two-point scattering amplitudes of two massless closed strings from a Dirichlet p-brane This construction relies on the observation that there is a simple relation between these D-brane amplitudes in type II superstring theory and four-point scattering amplitudes for type I open superstrings. From the two-point amplitudes, we derive the long range background fields for the D-branes, and verify that as expected they correspond to those of extremally charged p-brane solutions of the low energy effective action.