Short distance coefficients and the vanishing of the lepton asymmetry inB→Vl+l−

We derive a condition the short distance coefficients governing $\overrightarrow{b}\mathrm{}(s,d)\mathrm{}{\mathcal{l}}^{+}{\mathcal{l}}^{-}$ transitions must satisfy in order for the forward-backward asymmetry to vanish in the exclusive modes $\overrightarrow{B}{(K}^{*},\rho ){\mathcal{l}}^{+}{\mathcal{l}}^{-}.$ This relation, which is satisfied in the standard model, involves the coefficient entering in $\overrightarrow{b}s\gamma$ transitions as well as one of the additional Wilson coefficients present in the leptonic modes. We show that the resulting relation is largely free of hadronic uncertainties, thus constituting a reliable test of the standard model in exclusive rare $B$ decays.