Interference in the reaction $e^+e^-\to\gamma\pi^+\pi^-$ and the final state interaction

We describe the interference between amplitudes $e^ + e^-\to\rho\to\gamma\pi^+\pi^-$ and $e^ + e^-\to\phi\to\gamma (f_0 + \sigma) \to\gamma\pi^ + \pi^-$, with regard to the phase of the elastic $\pi\pi$ scattering background and mixing of the $f_0$ and $\sigma$ mesons. It is shown that the Fermi-Watson theorem for the final state interaction in the reaction $e^ + e^-\to\rho\to\gamma\pi^ + \pi^-$ is not valid in the case of soft photons, $\omega < 100$ MeV, in the $\phi$ meson region. The interference patterns in the spectrum of the photon energy differential cross-section and in the full cross-section as a function of the beam energy are obtained.