  Measurements of polarization fractions in $B \to VV$ transitions, with $V$ a
light vector meson, show that the longitudinal amplitude dominates in $B^0 \to
\rho^+ \rho^-$, $B^+ \to \rho^+ \rho^0$, and $B^+ \to \rho^0 K^{*+}$ decays and
not in the penguin induced decays $B^0 \to \phi K^{*0}$, $B^+ \to \phi K^{*+}$.
We study the effect of rescattering mediated by charmed resonances, finding
that in $B \to \phi K^*$ it can be responsible of the suppression of the
longitudinal amplitude. For the decay $B \to \rho K^*$ we find that the
longitudinal fraction cannot be too large without invoking new effects.
