Kepler-78 is host to a transiting 8.5-hour orbit super-Earth .
In this paper , the rotation and magnetic properties of the planet host star are studied .
We first revisit the Kepler photometric data for a detailed description of the rotation properties of Kepler-78 , showing that the star seems to undergo a cycle in the spot pattern of \sim 1,300 day duration .
We then use spectropolarimetric observations with CFHT/ESPaDOnS to measure the circular polarization in the line profile of the star during its rotation cycle , as well as spectroscopic proxies of the chromospheric activity .
The average field has a strength of 16 G. The magnetic topology is characterized by a poloidal and a toroidal component , encompassing 60 % and 40 % of the magnetic energy , respectively .
Differential rotation is detected with an estimated rate of 0.105 \pm 0.039 rad.d ^ { -1 } .
Activity tracers vary with the rotation cycle of the star ; there is no hint that a residual activity level is related to the planetary orbit at the precision of our data .
The description of the star magnetic field ’ s characteristics then may serve as input for models of interactions between the star and its close-by planet , e.g. , Ohmic dissipation and unipolar induction .