  Cosmic Rays (CR) are high energy particles which come from the universe. When
one of those particles enters to the atmosphere of the earth it produces an air
shower, conformed by secondary particles in which the initial energy is
distributed. The Pierre Auger Observatory, located in Argentina, is dedicated
to the study of those events. One of the main goals is to find out where those
CR are coming from and which kind of chemical composition do they have. In this
work we show the status of a study of the risetime as a function of the
distance to the shower core (near to the air axis of the shower) for different
zenith angles and energies, obtaining a new variable that will be compared with
other variables used by the Observatory. The main objective of this study is to
better understand risetime as a mass composition sensitive parameter of CR.
