  We report the measurement of the ratio $\Gamma(\eta \to
\pi^+\pi^-\gamma)/\Gamma(\eta \to \pi^+\pi^-\pi^0)$ analyzing a large sample of
$\phi \to \eta \gamma$ decays recorded with the KLOE experiment at the
DA$\Phi$NE $e^+ e^-$ collider, corresponding to an integrated luminosity of 558
pb$^{-1}$. The $\eta \to \pi^+\pi^-\gamma$ process is supposed to proceed both
via a resonant contribution, mediated by the $\rho$ meson, and a non resonant
direct term, connected to the box anomaly. The presence of the direct term
affects the partial width value. Our result $R_{\eta}=\Gamma(\eta \to \pi^+
\pi^- \gamma)/\Gamma(\eta \to \pi^+ \pi^- \pi^0)= 0.1838\pm 0.0005_{stat} \pm
0.0030_{syst}$ is in agreement with a recent CLEO measurement, which differs by
more 3 $\sigma$ from the average of previous results.
