On the photon constituency of protons

We argue that existing measurements of ep collisions at HERA-in which an en ergetic photon is made via a QED 'Compton' subprocess - can provide rather detailed information on the photonic parton density of the proton. This fun ction and its deviations from Bjorken scaling should be measurable, allowin g for an interesting test of the theory. The photonic distribution function and its gluonic counterpart should show a strikingly different evolution w ith momentum scale. (C) 1999 Published by Elsevier Science B.V. All rights reserved.