By examining in details the disk level S-matrix element of one massless RR and one NSNS states at order O( alpha ^2), we find the coupling of one RR and one NSNS fields on the world volume of a D _p -brane. The non-zero couplings involve the first derivative of the RR field strengths F^{(p)}, F^{(p+2)} and F^{(p+4)}. We then fix the on-shell ambiguity of the couplings by requiring consistency with the linear T-duality transformations. Moreover, consistency with the non-linear T-duality requires that the RR field strength in the above couplings to be { cal F}=d{ cal C} where { cal C}=e^{B}C.

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