Weak magnetic fields in CP stars

International audience; We present the first results of a magnetic survey of main sequence A and late B stars with NARVAL, the new generation spectropolarimeter available at Telescope Bernard Lyot at Pic du Midi Observatory. We observed 3 HgMn stars, 12 Am stars and the only Ap star for which no Zeeman detection was obtained with MuSiCoS. If we combine our results with those of our previous survey of weak magnetic fields in spectroscopically selected Ap/Bp stars we get the following situation: - Ap/Bp stars: all these stars appear to host a detectable magnetic field. A threshold dipolar magnetic field of 300 G appears to exist, below which fields are very rare and perhaps altogether absent. - Am and HgMn stars: no magnetic field is detected up to now for the 15 observed stars. An accuracy down to 0.3 G (respectively 1 G) is reached on longitudinal magnetic field measurements for Am (respectively HgMn) stars. We propose a possible interpretation of the observed threshold field which naturally explains the magnetic dichotomy: there exists a critical field strength above which stable magnetic configurations exist and below which any large scale field configuration is destroyed by some instability.