Two-photon exchange at low Q²

We study two-photon exchange for elastic electron-proton scattering at low Q^2. Compact approximate formulae for the amplitudes are obtained. Numerical calculations are done for Q^2 < 0.1 GeV^2 with several realistic form factor parameterizations, yielding similar results. They indicate that the corrections to magnetic form factor can visibly affect cross-section and proton radii. For low-Q^2 electron-neutron scattering two-photon exchange corrections are shown to be negligibly small.