Detector calibration, luminosity monitoring, and search for ane/emph>at DESY HERA

We propose eγ scattering, where the photons are quasireal ones generated by the proton beam, to be used for electromagnetic calibration of the apparatus and for a normalization of experimental luminosities at DESY HERA. The same process may also be used for searching for an $e^{\mathrm{*}}$, or determining a limit for its coupling constant, in particular, in the mass range of 100–200 GeV. We discuss experimental requirements and results to be anticipated.