Context : During the transition from the Asymptotic Giant Branch ( AGB ) to Planetary Nebulae ( PN ) , the circumstellar geometry and morphology change dramatically .
Another characteristic of this transition is the high mass loss rate , that can be partially explained by radiation pressure and a combination of various factors like the stellar pulsation , the dust grain condensation and opacity in the upper atmosphere .
The magnetic field can also be one of the main ingredients that shapes the stellar upper atmosphere and envelope .

Aims : Our main goal is to investigate for the first time the spatial distribution of the magnetic field in the envelope of IRC+10216 .
More generally we intend to determine the magnetic field strength in the circumstellar envelope ( CSE ) of C-rich evolved stars , compare this field with previous studies for O-rich stars , and constrain the variation of the magnetic field with r the distance to the star ’ s center .

Methods : We use spectropolarimetric observations of the Stokes V parameter , collected with Xpol on the IRAM-30m radiotelescope , observing the Zeeman effect in seven hyperfine components of the CN J = 1-0 line .
We use Crutcher ’ s method to estimate the magnetic field .
For the first time , the instrumental contamination is investigated , through dedicated studies of the power patterns in Stokes V and I in detail .

Results : For C-rich evolved stars , we derive a magnetic field strength ( B ) between 1.6 and 14.2 mG while B is estimated to be 6 mG for the proto-PN ( PPN ) AFGL618 , and an upper value of 8 mG is found for the PN NGC7027 .
These results are consistent with a decrease of B as 1/r in the environment of AGB objects , i.e. , with the presence of a toroidal field .
But this is not the case for PPN and PN stars .
Our map of IRC+10216 suggests that the magnetic field is not homogeneously strong throughout or aligned with the envelope and that the morphology of the CN emission might have changed with time .

Conclusions :