  We study the effects of nonzero photon momentum on the triply-differential
cross section for (\gamma,2e) processes. Due to the low value of the photon
momentum, these effects are weak and manifest only in special kinematical
conditions like the back-to-back emission of the electrons with equal energy
sharing. Helium and a few light helium-like ions are treated in detail. Quite
unexpectedly, the magnitude of these effects is maximal for relatively small
photon energies. However, although this effect on the TDCS remains rather
small, of the order of a few mbarn eV^{-1} sr^{-2}, it is sufficient to be
observed experimentally.
