Radiative Four--Meson Amplitudes in Chiral Perturbation Theory
  We present a general discussion of radiative four--meson processes to $O(p^4)$ in chiral perturbation theory. We propose a definition of ``generalized bremsstrahlung'' that takes full advantage of experimental information on the corresponding non--radiative process. We also derive general formulae for one--loop amplitudes which can be applied, for instance, to $\eta \ra 3\pi\gamma$, $\pi \pi \ra \pi \pi \gamma$ and $K \ra 3\pi\gamma$. 
G. D'Ambrosio, G. Ecker, G. Isidori and H. Neufeld