Context. Although the star itself is not helium enriched, the periodicity and the variability in the UV wind lines of the pulsating B1 IV star β Cephei are similar to what is observed in magnetic helium-peculiar B stars, suggesting that β Cep is magnetic. Aims. We searched for a magnetic field using high-resolution spectropolarimetry. From UV spectroscopy, we analysed the wind variability and investigated the correlation with the magnetic data. Methods. Using 130 time-resolved circular polarisation spectra that were obtained with the MuSiCoS échelle spectropolarimeter at the 2m Telescope Bernard Lyot from 1998, when β Cep was discovered to be magnetic, until 2005, we applied the least-squares deconvolution method on the Stokes V spectra and derived the longitudinal component of the integrated magnetic field over the visible hemisphere of the star. We performed a period analysis on the magnetic data and on equivalent-width measurements of UV wind lines obtained over 17 years. We also analysed the short-and long-term radial velocity variations, which are due to the pulsations and the 90-year binary motion, respectively. Results. β Cep hosts a sinusoidally varying magnetic field with an amplitude 97 ± 4 G and an average value −6 ± 3 G. From the UV wind line variability, we derive a period of 12.00075(11) days, which is the rotation period of the star, and is compatible with the observed magnetic modulation. Phases of maximum and minimum field match those of maximum emission in the UV wind lines, strongly supporting an oblique magnetic-rotator model. We discuss the magnetic behaviour as a function of pulsation behaviour and UV line variability. Conclusions. This paper presents the analysis of the first confirmed detection of a dipolar magnetic field in an upper main-sequence pulsating star. Maximum wind absorption originates in the magnetic equatorial plane. Maximum emission occurs when the magnetic north pole points to the Earth. Radial velocities agree with the ∼90-year orbit around its Be-star binary companion.

Abstract.We discuss the observational history of the Wolf-Rayet object WR 46 (WN3p), including a reinvestigation of the original discovery plates from early this century. We find that the reported presence of N iii lines is a mis-interpretation of N v lines and conclude that the object did not change its spectral type since the first recording one century ago. We performed photometric monitoring in the period 1986-1999, and confirm that the object shows cyclical variability on a time scale of hours. The shape of the light curves varies from purely sinusoidal to irregular, and from an amplitude of nearly 0. m 1 to constancy. In addition, night-to-night variability of the mean brightness causes folded light curves to display a large scatter. We investigate the frequency behaviour of the photometric data. From the periodograms of our two large data sets, in 1989 and in 1991, we identify frequencies of significantly different values 7.08 cd −1 and 7.34 cd −1 , respectively. Moreover, the 1989 data show strong evidence for an additional frequency fx = 4.34 cd −1 . The periodograms of our eight smaller data sets show more ambiguous behaviour. We discuss whether these latter data show evidence for multi-frequency behaviour, or whether they can be reconciled with a single clock with a changing clock-rate. As pointed out by van Genderen et al. (1991), if the data are folded using twice the single-wave period, the light curves appear ellipsoidal with unequal minima. Although the difference in depth of the minima is hardly significant, it does occur in both large data sets. Moreover, the simultaneously obtained radial velocity measurements are in better agreement with the double-wave than the single-wave period (Paper II). Finally, Marchenko et al. (2000) observed WR 46 to have a single-wave period of the same order as the double-wave period identified here. The periodograms of the (V -W ) colour index show that the colour changes are controlled by single-wave frequencies, or their sub-harmonics (double-wave periods), but not by fx. The colour variation of WR 46 is peculiar in the sense that the object is red when bright and blue when faint. Although the spectrum of WR 46 has been suggested to originate from a stellar disc, this peculiar colour behaviour is in line with its WR nature, which is also confirmed by its spectral variability (Marchenko et al. 2000; Paper II). In addition, our seasonal photometric averages of WR 46 show a rise from 1989 to 1991 of 0. m 12, confirming the brightening detected by the Hipparcos-satellite (Marchenko et al. 1998). Eventually, WR 46 brightened by about 0. m 25 and subsequently declined on a time scale of years. Such a rise is unique among the WR stars in the Hipparcos-survey, and has not been found anywhere else. We investigate the changes to the double-wave behaviour and mean colour-index coinciding with the period change and brightening. Interpretation of the object as either a multi-frequency non-radial WR pulsator, or a WR binary with possible large orbital decay is deferred to Pap...

New circular spectropolarimetric observations of the Bl Hie star j3 Cep (vsini = 25 km s _1 ) show a sinusoidally varying weak longitudinal magnetic field (~ 200 G peak-to-peak). The period corresponds to the 12 day period in the stellar wind variations observed in ultraviolet spectral lines. Maximum field occurs at 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 12 days.

Because of a small difference in the minima, this periodicity may be a double-wave phenomenon (P dw ). The line fluxes vary in concert with the magnitudes. The significant difference of the periods can be either due to the occurence of two distinct periods, or due to a gradual change of the periodicity. A gradual brightening of the system of 0.m 12 appeared to accompany the period change. In addition, the light variations in 1989 show strong evidence for an additional period Px = 0.2304 d. Generally, the radial velocities show a cyclic variability on a time scale of the photometric double-wave. However, often they do not vary at all. The observed variability confirms the Population I WR nature of the light source, as noted independently by Marchenko et al. (2000). In the present paper, we first show how the photometric double-wave variability can be interpreted as a rotating ellipsoidal density distribution in the stellar wind. Subsequently, we discuss what mechanisms could drive such a configuration. First, stellar rotation of a single star is discarded as a likely cause. Second, the obvious interpretation of the double-wave photometry, i.e., a close binary system, is investigated. However unlikely, we discuss how the observed period change might be reconciled within a model of a strongly interacting binary. Third, an interpretation of a non-radial multi-mode pulsator is investigated. The observed period change and the multi-frequency behaviour do support this interpretation. We propose that the pulsational mode l = 1 and |m| = 1 may mimic a "binary" light-and radial-velocity curve. However, the phasing of the radial velocity and the light curve may be inconsistent. The possibility l = 2 and |m| = 0 is also discussed. Finally, we suggest how the enigma of the variability of WR 46 may be solved.