BPS States on a Three Brane Probe
  Recently Banks, Douglas and Seiberg have shown that the world volume theory of a three brane of the type IIB theory in the presence of a configuration of four Dirichlet seven branes and an orientifold plane is described by an N=2 supersymmetric SU(2) gauge theory with four quark flavours in 3+1 dimensions. In this note we show how the BPS mass formula for N=2 supersymmetric gauge theory arises from masses of open strings stretched between the three brane and the seven brane along appropriate geodesics. 
Ashoke Sen