A method to estimate primary composition with the Auger Observatory

We explore the feasibility of estimating primary cosmic ray composition at ultra high energies from the sum of muon, electron and photon densities and the depth of maximum of extensive air showers detected by the Auger Observatory. From the information of the Fluorescence ($X_{max}$) and water Cerenkov detectors ($\rho(1000)$) we infere the most probable type of primary which originated the shower. The method is tested simulating Extensive Air Showers (EAS) at energies up to $10^{20}$ eV using a version of MOCCA/Sibyll adapted for Dec Alpha Servers. We also discuss some results on the mass composition using the depth of the shower maximum and the lateral distribution of showers in the energy range $10^{18}$-$10^{20}$ eV.