  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.
