Study of the Cosmic Ray Composition above 0.4 EeV using the Longitudinal Profiles of Showers observed at the Pierre Auger Observatory

The Pierre Auger Observatory has been collecting data in a stable manner since January 2004. We present here a study of the cosmic ray composition using events recorded in hybrid mode during the first years of data taking. These are air showers observed by the fluorescence detector as well as the surface detector, so the depth of shower maximum, Xmax, is measured directly. The cosmic ray composition is studied in different energy ranges by comparing the observed average Xmax with predictions from air shower simulations for different nuclei. The change of <Xmax> with energy (elongation rate) is used to derive estimates of the change in primary composition.