Recently, it has been observed that the kinematic factor of the disk-level S-matrix element of two RR two-forms and the disk-level S-matrix element of two B-fields on the world volume of D$_3$-brane are compatible with the standard rules of S-duality. Inspired by this observation, we speculate that the S-matrix elements on the world volume of D$_3$-brane are invariant under the $SL(2,Z)$ transformation. Compatibility with S-duality requires the S-matrix elements on the world volume of D$_1$-brane and D$_5$-brane to be extended to the $SL(2,Z)$-covariant form. In particular, this implies the S-matrix elements on the world volume of F$_1$-string and NS$_5$-brane at strong coupling to be related by S-duality to the disk-level S-matrix elements of D$_1$-string and D$_5$-brane, respectively. %The S-matrix elements then dictate that both F$_1$-string and NS$_5$-brane have D$_1$-string excitations. The contact terms of these S-matrix elements at order $O(\alpha'^0)$ produce a Born-Infeld and Chern-Simons type effective action for both F$_1$-string and NS$_5$-brane. They are consistent with the $SL(2,R)$-covariant action appears in the literature.

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