  Although recent measurements of the shower profiles of ultra-high energy
cosmic rays suggest that they are largely initiated by heavy nuclei, such
conclusions rely on hadronic interaction models which have large uncertainties.
We investigate an alternative test of cosmic ray composition which is based on
the observation of ultra-high energy photons produced through cosmic ray
interactions with diffuse low energy photon backgrounds during intergalactic
propagation. We show that if the ultra-high energy cosmic rays are dominated by
heavy nuclei, the flux of these photons is suppressed by approximately an order
of magnitude relative to the proton-dominated case. Future observations by the
Pierre Auger Observatory may be able to use this observable to constrain the
composition of the primaries, thus providing an important cross-check of
hadronic interaction models.
