  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.
