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We have obtained high-resolution spectropolarimetric data for the planet-hosting star τ Bootis, using the ESPaDOnS spectropolarimeter at the Canada–France–Hawaii Telescope (CFHT). A weak but clear Stokes V signature is detected on three of the four nights of 2006 June during which we have recorded data. This polarimetric signature indicates with no ambiguity the presence of a magnetic field at the star's surface, with intensity of just a few gauss. The analysis of the photospheric lines of τ Boo at ultra-high signal-to-noise ratio reveals the presence of an 18 per cent relative differential rotation. Tentative Zeeman–Doppler imaging, using our spectropolarimetric observations covering only a fraction of the star's rotational phase, indicates a magnetic field with a dominant potential field component. The data are best fitted when a 3.1-d period of modulation and an intermediate inclination are assumed. Considering the level of differential rotation of τ Boo, this implies a rotation period of 3.0 d at the equator and of 3.7 d at the pole, and a topology of the magnetic field where its main non-axisymmetric part is located at low latitudes. The planet is probably synchronized with the star's rotation at intermediate latitudes, while the non-axisymmetric part of the magnetic field seems located at lower latitudes. Our limited data do not provide sufficient constraints on the magnetic field to study a possible interaction of the planet with the star's magnetosphere. Investigating this issue will require data with much better phase coverage. Similar studies should also be performed for other stars hosting close-in giant planets. [ABSTRACT FROM AUTHOR]

From observations made with the ESPaDOnS spectropolarimeter, recently installed on the 3.6-m Canada–France–Hawaii Telescope, we report the discovery of a strong magnetic field in the Of?p spectrum variable HD 191612 – only the second known magnetic O star (following θ1 Ori C). The stability of the observed Zeeman signature over four nights of observation, together with the non-rotational shape of line profiles, argues that the rotation period of HD 191612 is significantly longer than the 9-d value previously proposed. We suggest that the recently identified 538-d spectral variability period is the rotation period, in which case the observed line-of-sight magnetic field of implies a large-scale field (assumed dipolar) with a polar strength of about −1.5 kG. If confirmed, this scenario suggests that HD 191612 is, essentially, an evolved version of the near-zero-age main-sequence magnetic O star θ1 Ori C, but with an even stronger field (about 15 kG at an age similar to that of Ori C). We suggest that the rotation rate of HD 191612, which is exceptionally slow by accepted O-star standards, could be due to angular momentum dissipation through a magnetically confined wind. [ABSTRACT FROM AUTHOR]