  We report a calculation of the expected rate of inclined air showers induced
by ultra high energy cosmic rays to be obtained by the Auger Southern
Observatory assuming different mass compositions. We describe some features
that can be used to distinguish photons at energies as high as 10$^{20}$ eV.
The discrimination of photons at such energies will help to test some models of
the origin of ultra-high-energy cosmic rays.
