  The beam-spin asymmetry in hard electroproduction of photons has been
measured for the first time. The data have been accumulated by the HERMES
experiment at DESY using the HERA 27.6 GeV longitudinally polarized positron
beam and an unpolarized hydrogen gas target. The asymmetry in the azimuthal
distribution of the produced photons in the angle phi relative to the lepton
scattering plane was determined with respect to the helicity state of the
incoming positron beam. The beam-spin analyzing power in the sin(phi) moment
was measured to be -0.23 +/- 0.04(stat) +/- 0.03(syst) in the missing-mass
range below 1.7 GeV. The observed asymmetry is attributed to the interference
of the Bethe-Heitler and deeply-virtual Compton scattering processes.
