  Using the explicit string scattering calculation and the linear T-dual ward
identity, we evaluate the string theory disc amplitude of one Ramond-Ramond
field $C^{(p+1)}$ and two Neveu-Schwarz B-fields in the presence of a single
$D_p$-brane in type $IIB$ string theory. From this amplitude we extract the
$O(\alpha'^2)$ (or equivalently four-derivative) part of the $D_p$-brane
couplings involving these fields.
