Context : With a Jupiter-mass planet orbiting at a distance of only 0.031 AU , the active K2 dwarf HD 189733 is a potential candidate in which to study the magnetospheric interactions of a cool star with its recently-discovered close-orbiting giant planet .

Aims : We decided to explore the strength and topology of the large-scale magnetosphere of HD 189733 , as a future benchmark for quantitative studies for models of the star/planet magnetic interactions .

Methods : To this end , we used ESPaDOnS , the new generation spectropolarimeter at the Canada-France-Hawaii 3.6m telescope , to look for Zeeman circular polarisation signatures in the line profiles of HD 189733 in 2006 June and August .

Results : Zeeman signatures in the line profiles of HD 189733 are clearly detected in all spectra , demonstrating that a field is indeed present at the surface of the star .
The Zeeman signatures are not modulated with the planet ’ s orbital period but apparently vary with the stellar rotation cycle .
The reconstructed large-scale magnetic field , whose strength reaches a few tens of G , is significantly more complex than that of the Sun ; it involves in particular a significant toroidal component and contributions from magnetic multipoles of order up to 5 .
The Ca ii H & K lines clearly feature core emission , whose intensity is apparently varying mostly with rotation phase .
Our data suggest that the photosphere and magnetic field of HD 189733 are sheared by a significant amount of differential rotation .

Conclusions : Our initial study confirms that HD 189733 is an optimal target for investigating activity enhancements induced by closely orbiting planets .
More data are needed , densely covering both the orbital and rotation cycles , to investigate whether and how much the planet contributes to the overall activity level of HD 189733 .