  Among the main sequence intermediate mass A and B stars, around 5% host
large-scale organized magnetic fields. Most of these stars are very slow
rotators compared to their non-magnetic counterparts, and show photospheric
abundance anomalies. They are referred to as the Ap/Bp stars. One of the
greatest challenges, today is to understand the origin of their magnetic field
and their slow rotation. The favoured hypothesis is a fossil origin of the
magnetic field, in which the magnetic fields of Ap/Bp stars are relics of those
which existed in the parental molecular clouds during the formation. This
implies that the magnetic field must survive all the initial phases of the
stellar evolution and especially the pre-main sequence (PMS) phase. This is
consistent with the general belief that magnetic braking occurs during the PMS
phase, which sheds angular momentum and slows the rotation of these stars. In
this context, we proceeded with a survey of a sample of around 50 PMS Herbig
Ae/Be stars, using the new spectropolarimeter ESPaDOnS at the CFHT, in order to
study the magnetic field and the rotation velocity of these stars. This talk
reviews the results of our survey, as well as their consequences for the origin
of the magnetic fields and the evolution of the rotation of intermediate mass
stars during the PMS phase.
