Radiative Leptonic Decays of D_s^\pm and D^\pm Mesons
  In this work, we investigate the radiative leptonic decays D_(s)^- \to \gamma \ell \bar\nu (\ell = e, \mu) at tree level within the non-relativistic consistuent quark model and the effective Lagrangian for the heavy flavor decays. We find that the contributions for the three Feynman diagrams are all important. With the full calculation, the decay branching ratios are of the order of 10^-5 for D_s^- \to \gamma \ell \bar\nu and 10^-6 for D^- \to \gamma \ell \bar\nu, respectively. These decays can be measured at B factories and future CLEO-C experiments to determine the decay constants f_D_s and f_D. 
Cai-Dian Lu, and Ge-Liang Song