Evelyne Alecian (RMC) Title: Study of the magnetism and rotation of Herbig Ae/Be stars with ESPaDOnS Abstract: Today, one of the greatest challenges in stellar physics is to understand the origin of the magnetic fields and slow rotation of chemically peculiar Ap/Bp stars. The favoured theory for the origin of their magnetic fields is the fossil field hypothesis, which proposes that the fields are relics from those which were present in their parental molecular clouds. This hypothesis implies that magnetic fields should be present at all stages of the evolution of these stars, in particular during the pre-main sequence (PMS) phase. Moreover, the leading theory explaining the slow rotation of the Ap/Bp stars requires the presence of strong magnetic fields at the stellar surface during the PMS phase, to couple to the accretion disk and generate magnetic braking. Until recently, however, no information has been available about the magnetic or rotational properties of the PMS A and B type stars. The new high-resolution spectropolarimeter ESPaDOnS, installed in 2005 at CFHT, provided us with the capability to investigate magnetism and rotation in a large sample of PMS A/B stars - the so-called Herbig Ae/Be stars. These observations have allowed us to perform robust and detailed tests of the fossil field hypothesis. In this talk, I will present the results of 3 years of observations with ESPaDOnS, as well as their implications for understanding the magnetism and rotation of intermediate-mass stars.