The prodigious mass loss observed in the numerous and widespread evolved stars make these objects the main recycling agents of the interstellar medium, and thus one of the most important objects in the Universe. During its quick transition to the Planetary Nebula, the Asymptotic Giant Branch star will completely change its geometry. This AGB stellar evolution stage is characterized by a high mass loss driven by the radiation pressure. Strong magnetic field may rule the mass loss geometry; in particular, it could be the cause of a higher or lower mass-loss rate in the equatorial plane (Soker 2002), and thus determine the global shaping of these objects. Recently, magnetic field was discovered for the first time in central stars of PN (Jordan et al. 2005) and estimated to be at the kiloGauss level, but polarimetric observations toward AGB stars are needed to constrain the magnetic field strength. Herpin et al. (2006) have studied the polarization of the SiO maser emission in a representative sample of O-rich evolved stars and derived an estimate of the strength of the magnetic field, and hence determine its influence on evolved stars. The averaged magnetic field was estimated between 0 and 18 Gauss, with a mean value of 3.5 G; the field may follow a $1/r$ law throughout the circumstellar envelope. As a consequence, the magnetic field may have a real influence on the life of these evolved objects: it could play the role of a collimating, and more generally of a shaping, agent in evolved objects.

The same type of study should also be conducted within C-rich objects and PPN/PN objects to obtain unbiased conclusions. Using Xpol at the IRAM-30m telescope(Thum et al. 2008), we have conducted CN N=1-0 observations toward C-rich AGB stars, PPN and PN objects to investigate the Zeeman effect in this molecule and draw conclusion on the evolution of the magnetic field and its influence during the transition of an AGB star to the PN stage. Following the analysis procedure described by Crutcher et al. (1996) we derive an estimate of the magnetic field.