  We have obtained high resolution spectropolarimetric data for the
planet-hosting star $\tau$ Bootis, using the ESPaDOnS spectropolarimeter at
CFHT. A weak but clear Stokes $V$ signature is detected on three of the four
nights of June 2006 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 $\tau$ Boo at ultra-high
signal-to-noise reveals the presence of an 18% 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 3.1d period of modulation and an intermediate inclination are assumed.
Considering the level of differential rotation of $\tau$ Boo, this implies a
rotation period of 3.0d at the equator and of 3.7d 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 synchronised 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.
