On the photon constituency of protons

We argue that existing measurements of ep collisions at HERA --in which an energetic photon is made via a QED `Compton' subprocess-- can provide rather detailed information on the photonic parton density of the proton. This function and its deviations from Bjorken scaling should be measurable, allowing for an interesting test of the theory. The photonic distribution function and its gluonic counterpart should show a strikingly different evolution with momentum scale.