Virtual D-Branes
  Using a formalism developed to include collective coordinates, we calculate the contributions to S-matrix elements due to off-shell D-branes in a string coupling expansion. The formalism is further used to establish a two-dimensional computation of higher order corrections to the D-brane tension, both for the bosonic and the supersymmetric cases. 
David Berenstein, Richard Corrado, Willy Fischler, Sonia Paban, and   Moshe Rozali