P. Bickert, S. Scherer

We present a calculation of the decays ${\eta}^{{(}^{\prime})}\to {\pi}^{+}{\pi}^{-}{\gamma}^{(*)}$ at the one-loop level up to and including next-to-next-to-leading order (NNLO) in large-${N}_{c}$ chiral perturbation theory. The numerical evaluation of the results is performed successively at LO, NLO, and NNLO, fitting the relevant low-energy constants to the available experimental data. We discuss the widths and decay spectra of ${\eta}^{{(}^{\prime})}\to {\pi}^{+}{\pi}^{-}\gamma $ as well as ${\eta}^{{(}^{\prime})}\to {\pi}^{+}{\pi}^{-}{l}^{+}{l}^{-}$, with $l=e$, $\mu $.