Context :

Aims : We investigate the long-term evolution of the large-scale photospheric magnetic field geometry of the solar-type star HD 190771 .
With fundamental parameters very close to those of the Sun except for a shorter rotation period of 8.8 d , HD 190771 provides us with a first insight into the specific impact of the rotation rate in the dynamo generation of magnetic fields in 1 M _ { \odot } stars .

Methods : We use circularly polarized , high-resolution spectra obtained with the NARVAL spectropolarimeter ( Observatoire du Pic du Midi , France ) and compute cross-correlation line profiles with high signal-to-noise ratio to detect polarized Zeeman signatures .
From three phase-resolved data sets collected during the summers of 2007 , 2008 , and 2009 , we model the large-scale photospheric magnetic field of the star by means of Zeeman-Doppler imaging and follow its temporal evolution .

Results : The comparison of the magnetic maps shows that a polarity reversal of the axisymmetric component of the large-scale magnetic field occurred between 2007 and 2008 , this evolution being observed in both the poloidal and toroidal magnetic components .
Between 2008 and 2009 , another type of global evolution occured , characterized by a sharp decrease of the fraction of magnetic energy stored in the toroidal component .
These changes were not accompanied by significant evolution in the total photospheric magnetic energy .
Using our spectra to perform radial velocity measurements , we also detect a very low-mass stellar companion to HD 190771 .

Conclusions :