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Using a general parametrization of the spin structure of the matrix elementfor the elastic $e^-+^4He$ scattering and for the annihilation $e^++e^-\to\pi^++\pi^-$ reactions in terms of two complex amplitudes, we derive generalproperties of the observables in presence of two--photon exchange. We show thatthis mechanism induces a specific dependence of the differential cross sectionon the angle of the emitted particle. We reanalyze the existing experimentaldata on the differential cross section, for elastic electron scattering on$^4He$, in the light of this result.

General analysis of two--photon exchange in elastic electron--$^4He$ scattering and $e^++e^-\to \pi^++\pi^-.$