  Sum rules for exclusive production of heavy meson pairs in $e^+e^-$
annihilation are used to evaluate the $\Upsilon(4S) \to B \bar B$ and
$\psi(3770) \to D \bar D$ decay widths. Infinitely heavy quark limit is
discussed, so that scaling law for the quarkonium-meson coupling constant is
derived. A value of the $B\bar B$ pair contribution into the leptonic constant
$f_{\Upsilon(4S)}$ is estimated.
