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Aims: We observe the nearby, weakly-active single giant, Pollux, in order todirectly study and infer the nature of its magnetic field. Methods: We used thenew generation spectropolarimeters ESPaDOnS and NARVAL to observe and detectcircular polarization within the photospheric absorption lines of Pollux. Ourobservations span 18 months from 2007-2009. We treated the spectropolarimetricdata using the Least-Squares Deconvolution method to create highsignal-to-noise ratio mean Stokes V profiles. We also measured the classicalactivity indicator S-index for the Ca H&K lines, and the stellar radialvelocity (RV). Results: We have unambiguously detected a weak Stokes V signalin the spectral lines of Pollux, and measured the related surface-averagedlongitudinal magnetic field Bl. The longitudinal field averaged over the spanof the observations is below one gauss. Our data suggest variations of thelongitudinal magnetic field, but no significant variation of the S-index. Weobserve variations of RV which are qualitatively consistent with the publishedephemeris for a proposed exoplanet orbiting Pollux. The observed variations ofBl appear to mimic those of RV, but additional data for this relationship to beestablished. Using evolutionary models including the effects of rotation, wederive the mass of Pollux and we discuss its evolutionary status and the originof its magnetic field. Conclusions: This work presents the first directdetection of the magnetic field of Pollux, and demonstrates that ESPaDOnS andNARVAL are capable of obtaining sub-G measurements of the surface-averagedlongitudinal magnetic field of giant stars, and of directly studying therelationships between magnetic activity, stellar evolution and planet hostingof these stars.

Discovery of a weak magnetic field in the photosphere of the single giant Pollux