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The importance of two-photon exchange in elastic electron-proton scattering was investigated by measuring the ratio of positron-proton to electron-proton scattering. Four-momentum transfers as large as 0.756 (BeV/c) 2 (19.5 F~2) were used. The data indicate that two-photon effects are (4.0dbl.5)% larger than those predicted by the radiative corrections at the highest momentum transfers attained in these experiments. The two-photon corrections predicted using a static charge distribution fit the data well at lower momentum transfers and forward angles, but appear to be small at higher momentum transfers and backward angles.

W e present evidence for a large scalar contribution to the cross section for the reaction ep +eK 'A. No evidence for a scalar contribution is found for the reaction ep i eK +XO. This is reminiscent o f the results for the ~+ n and r + A O final states.