We present an improved light-cone sum rule analysis of the decay form factors of $D$ and $D_s$ into $eta$ and $eta^{prime}$ and argue that these decays offer a very promissing possibility to determine the leading Fock-state gluonic contribution of the $eta$ at future experimental facilities as FAIR or Super-KEKB. We also give the corresponding branching ratios for B decays.
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