On modified tachyon DBI action
  Recently a modification of the tachyon DBI action has been proposed in which the tachyon carries the internal CP matrix \sigma_1 and \sigma_2. In this paper, we find the momentum expansion of the disk level S-matrix element of four tachyons and one gauge field in superstring theory and show that the first and second order terms of the expansion are in perfect agreement with the above tachyon DBI action. 
Mohammad R. Garousi