D-brane boundary states for type II superstrings are constructed by enforcing the conditions that preserve half of the space-time supersymmetry. A light-cone coordinate frame is used where time is identified as one of the coordinates transverse to the brane's (euclidean) world-volume so that the p-brane is treated as a (p + 1)-instanton. The boundary states have the superspace interpretation of top or bottom states in a light-cone string superfield. The presence of a non-trivial open-string boundary condensate give rise to the familiar D-brane source terms that determine the (linearized) Born-Infeld-like effective actions for p-branes and the (linearized) equations of motion for the massless fields implied by the usual p-brane ansätze. The 'energy' due to closed string exchange between separate D-branes is calculated (to lowest order in the string coupling) in situations with pairs of parallel, intersecting as well as orthogonal branes - in which case the unbroken supersymmetry may be reduced. Configurations of more than two branes are also considered in situations in which the supersymmetry is reduced to 1/8 or 1/16 of the full amount. The Ward identities resulting from the non-linearly realized broken space-time supersymmetry in the presence of a D-brane are also discussed.
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