  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.
