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The recent precision measurement of the relative yield of $D^+D^-$ and $D^0\bar D^0$ meson pairs at the $\psi(3770)$ peak in $e^+e^-$ annihilation isabout 3.7 sigma away from a straightforward theoretical estimate. It is arguedthat combined with an even more significant deviation of similar ratio for the$B$ mesons at the $\Upsilon(4S)$ peak, this provides an insight into thestructure of heavy mesons and requires both a form factor, corresponding totheir finite `size', and a strong interaction between the mesons mesons in theisovector state $I^G(J^P) = 1^+(1^-)$. It is also argued that the data for the$D$ and $B$ mesons are possibly compatible with the heavy quark limit of thesame meson-antimeson strong interaction in both systems.

Charged-to-neutral meson yield ratio at $\psi(3770)$ and $\Upsilon(4S)$ revisited