Meson loops and the $g_{D^* D \pi}$ coupling
  The $D^*D\pi$ form factor is evaluated at low and moderate $Q^2$ in a hadronic loop model, for off-shell $D$ mesons. The results contain arbitrary constants, which are fixed so as to match previous QCD sum rule calculations valid at higher $Q^2$. The form factor obtained in this way was used to extract the coupling constant, which is in very good agreement with the experimental value. 
F.O. Dur\~aes, F.S. Navarra, M. Nielsen and M.R. Robilotta