Box Anomaly and $\eta' \to \pi ^+ \pi ^- \gamma$ Decay
  In the framework of the "current scheme" of $\eta - \eta'$ mixing we calculate the amplitude of the decay $\eta' \to \pi ^+ \pi ^- \gamma $ in the soft limit and compare our results with recently reported values of the phenomenological additive terms to the $\rho $--meson background. 
A.V. Kisselev and V.A. Petrov