Leptonic decays of D_s and B^+ mesons in supersymmetric standard model with R-parity violating interactions

We investigate leptonic decays $D_s \to \tau \nu_\tau$ and $B^+ \to \tau \nu_\tau$ in R-parity violating (RPV) supersymmetric standard model. Taking account of interference between the s-channel slepton exchange and the $t$-channel squark exchange diagrams, we find that the supersymmetric contributions are cancelled between two diagrams so that the RPV couplings could be sizable under the experimental bounds. Constraints on the relative sign between the RPV couplings in $s$- and $t$-channel diagrams are also discussed.