  We present a new method to derive an upper limit on the ultra-high-energy
(UHE) photon flux above a given energy, using the Auger SD events. As previous
studies, it takes advantage of the fundamental differences between showers
induced by hadrons and showers induced by photons (signal rise time, slope of
the Lateral Distribution Function, radius of curvature), but it uses photon
shower simulations adapted to the actually observed SD events, on an
event-by-event basis. In particular, we find that a limited number of
simulations per event allows to reach an upper limit that is very close to the
ideal case that would be obtained with an infinite number of simulations. For
instance, 10 simulated showers per event are sufficient to reach an upper limit
only 10 percent higher than the ideal one. As an illustration, we apply this
method to the SD events reconstructed above 55 EeV, which provides an optimized
upper limit on the photon flux above 150 EeV, and indicate how this can be
easily extended to the lower-energy events, to constrain the photon flux down
to much lower energies.
