Discovery of a magnetic field in the Slowly Pulsating B star Cassiopeiae

(zeta)Cas is a B2 IV star with ...km s -1. Time-resolved circular spectropolarimetric observations of (zeta)Cas obtained in 2001 and 2002 with the Musicos échelle spectropolarimeter at the 2 m Télescope Bernard Lyot (TBL) show a sinusoidally varying longitudinal magnetic field with a strength between 10 G and -46 G for the averaged line-of-sight component, corresponding to ...G. The period corresponds very accurately with the 5.37045 day period as derived from stellar wind variations observed in the ultraviolet. The epoch of the positive maximum field corresponds in phase with the maximum emission in the UV wind lines. This gives compelling evidence for a magnetic rotator model for this star, with an unambiguous rotation period of 5.37 days. We searched for periodicity in line-profile variations (lpv), radial velocity and minimum intensity curves in the ~400 optical spectra. We found a non-radial pulsation mode with l=2+-1 at the frequency f=0.64 c d ^-1. From this periodicity and from stellar parameters derived from model fits, we propose to classify (zeta)Cas as a Slowly Pulsating B (SPB) star. This is the third detection of a magnetic field in an early B-type pulsating star and the first one in a SPB star.