We explicitly calculate the disk-level S-matrix element of two closed string R-R and one open string NS vertex operators in RNS formalism. We show that the amplitude satisfies various duality Ward identities. In particular, when one of the R-R is zero-form, the other one is two-form and the NS state is gauge boson, the amplitude transforms under S-duality Ward identity to the amplitude of one dilaton, one B-field and one gauge boson which has been recently calculated explicitly. We have also proposed a soft theorem for disk-level scattering amplitude of arbitrary number of hard closed strings and one soft open string at the leading order of soft momentum, and shown that the above amplitude satisfies the soft theorem.

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