  We report the detection of a magnetic field on the Of?p star HD108.
Spectropolarimetric observations conducted in 2007, 2008 and 2009 respectively
with NARVAL@TBL and ESPaDOnS@CFHT reveal a clear Zeeman signature in the
average Stokes V profile, stable on timescales of days to months and slowly
increasing in amplitude on timescales of years. We speculate that this
timescale is the same as that on which Ha emission is varying and is equal to
the rotation period of the star. The corresponding longitudinal magnetic field,
measured during each of the three seasons, increases slowly from 100 to 150G,
implying that the polar strength of the putatively-dipolar large-scale magnetic
field of HD108 is at least 0.5kG and most likely of the order of 1-2 kG. The
stellar and wind properties are derived through a quantitative spectroscopic
analysis with the code CMFGEN. The effective temperature is difficult to
constrain because of the unusually strong HeI4471 and HeI5876 lines. Values in
the range 33000-37000 K are preferred. A mass loss rate of about 1e-7 Msun/yr
(with a clumping factor f=0.01) and a wind terminal velocity of 2000 km/s are
derived. The wind confinement parameter eta_star is larger than 100, implying
that the wind of HD108 is magnetically confined. Stochastic short-term
variability is observed in the wind-sensitive lines but not in the photospheric
lines, excluding the presence of pulsations. Material infall in the confined
wind is the most likely origin for lines formed in the inner wind.
Wind-clumping also probably causes part of the Ha variability. The projected
rotational velocity of HD108 is lower than 50 km/s, consistent with the
spectroscopic and photometric variation timescales of a few decades. Overall,
HD108 is very similar to the magnetic O star HD191612 except for an even slower
rotation.
