Light-cone sum rules for the $D_{(s)}\rightarrow \eta^{(\prime)} l \nu_l$ form factor

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.