  Context: Recent spectropolarimetric observations of Herbig Ae/Be stars have
yielded new arguments in favour of a fossil origin for the magnetic fields of
intermediate mass stars. Aims: To study the evolution of these magnetic fields,
and their impact on the evolution of the angular momentum of these stars during
the pre-main sequence phase, we observed Herbig Ae/Be members of young open
clusters of various ages. Methods: We obtained high-resolution
spectropolarimetric observations of Herbig Ae/Be stars belonging to the young
open clusters NGC 6611 (< 6 Myr), NGC 2244 (~1.9 Myr), and NGC 2264 (~8 Myr),
using ESPaDOnS at theCanada-France-Hawaii Telescope. Results: Here we report
the discovery of strong magnetic fields in two massive pre-main sequence
cluster stars. We detected, for the first time, a magnetic field in a pre-main
sequence rapid rotator: the 10.2 Msun Herbig B1.5e star W601 (NGC 6611; v sin i
~ 190 km/s). Our spectropolarimetric observations yield a longitudinal magnetic
field larger than 1 kG, and imply a rotational period shorter than 1.7 days.
The spectrum of this very young object (age ~ 0.017 Myr) shows strong and
variable lines of He and Si. We also detected a magnetic field in the 12.1 Msun
B1 star OI 201 (NGC 2244; v sin i = 23.5 km/s). The Stokes V profile of this
star does not vary over 5 days, suggesting a long rotational period, a pole-on
orientation, or aligned magnetic and rotation axes. OI 201 is situtated near
the Zero-Age Main Sequence on the HR diagram, and exhibits normal chemical
abundances and no spectrum variability.
