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Aims. We investigate the long-term evolution of the large-scale photosphericmagnetic field geometry of the solar-type star HD 190771. With fundamentalparameters very close to those of the Sun except for a shorter rotation periodof 8.8 d, HD 190771 provides us with a first insight into the specific impactof the rotation rate in the dynamo generation of magnetic fields in 1 $M_\odot$stars.  Methods. We use circularly polarized, high-resolution spectra obtained withthe NARVAL spectropolarimeter (Observatoire du Pic du Midi, France) and computecross-correlation line profiles with high signal-to-noise ratio to detectpolarized Zeeman signatures. From three phase-resolved data sets collectedduring the summers of 2007, 2008, and 2009, we model the large-scalephotospheric magnetic field of the star by means of Zeeman-Doppler imaging andfollow its temporal evolution.  Results. The comparison of the magnetic maps shows that a polarity reversalof the axisymmetric component of the large-scale magnetic field occurredbetween 2007 and 2008, this evolution being observed in both the poloidal andtoroidal magnetic components. Between 2008 and 2009, another type of globalevolution occured, characterized by a sharp decrease of the fraction ofmagnetic energy stored in the toroidal component. These changes were notaccompanied by significant evolution in the total photospheric magnetic energy.Using our spectra to perform radial velocity measurements, we also detect avery low-mass stellar companion to HD 190771.

A polarity reversal in the large-scale magnetic field of the rapidly rotating Sun HD 190771