QED corrections in radiative return method at measurement of the $e^+e^-\to \pi^+\pi^-$ cross–section below 1 GeV

We discuss radiative corrections at the measurement of the $e^+e^-\to \pi^+\pi^-$ cross-section by the radiative return method without photon tagging. The radiative corrections to the initial–state radiation process are computed for DA$\Phi$NE conditions, using the quasi–real electron approximation for both, the cross-section and the underlying kinematics. The efficiency of experimental rules for event selection by the restrictions on the lost invariant mass and so-called «track mass» is estimated. Some numerical calculations illustrate our analytical results.