  Making use of the measurement of the $B\to K^*\gamma$ branching ratio
together with the relations following from the limit of high recoil energy, we
obtain stringent constraints on the values of the form-factors entering in
heavy-to-light $B\to V\ell\ell'$ processes such as $B\to K^*\ell^+\ell^-$,
$B\to K^*\nu \bar{\nu}$ and $B\to \rho\ell\nu$ decays. We show that the
symmetry predictions, when combined with the experimental information on
radiative decays, specify a severely restricted set of values for the vector
and axial-vector form-factors evaluated at zero momentum transfer, $q^2=0$.
These constraints can be used to test model calculations and to improve our
understanding of the $q^2$-dependence of semileptonic form-factors. We stress
that the constraints remain stringent even when corrections are taken into
account.
