  We present a combined fit of a simple astrophysical model of UHECR sources to
both the energy spectrum and mass composition data measured by the Pierre Auger
Observatory. The fit has been performed for energies above 5 EeV, i.e. the
region of the all-particle spectrum above the so-called "ankle" feature. The
astrophysical model we adopted consists of identical sources uniformly
distributed in a comoving volume, where nuclei are accelerated with a
rigidity-dependent mechanism. The fit results suggest sources characterized by
relatively low maximum injection energies and hard spectral indices. The impact
of various systematic uncertainties on the above result is discussed.
