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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 isdiscussed, so that scaling law for the quarkonium-meson coupling constant isderived. A value of the $B\bar B$ pair contribution into the leptonic constant$f_{\Upsilon(4S)}$ is estimated.Comment: 14 pages, 1 Fig., Standard LATE

SCALING RELATION FOR THE $\Upsilon (4S)\to B\bar{B}$ AND $\psi (3770)\to D\bar{D}$ DECAY CONSTANTS FROM PHENOMENOLOGICAL SUM RULES