  From direct observations of the longitudinal development of ultra-high energy
air showers performed with the Pierre Auger Observatory, upper limits of 3.8%,
2.4%, 3.5% and 11.7% (at 95% c.l.) are obtained on the fraction of cosmic-ray
photons above 2, 3, 5 and 10 EeV (1 EeV = 10^18 eV) respectively. These are the
first experimental limits on ultra-high energy photons at energies below 10
EeV. The results complement previous constraints on top-down models from array
data and they reduce systematic uncertainties in the interpretation of shower
data in terms of primary flux, nuclear composition and proton-air
cross-section.
