2007A&A...472..649D

For the past 3 years we have been monitoring a sample of 62 K giant stars using precise stellar radial velocity measurements with high accuracy taken at the Thueringer Landessternwarte Tautenburg. We search for sub-stellar companions to giant stars and try to understand the nature of the diverse radial velocity variations exhibited by K giant stars. We present precise stellar radial velocity measurements of the K1III giant star 4 UMa (HD 73108). These were obtained using the coude echelle spectrograph of 2-m Alfred Jensch Telescope. The wavelength reference for the radial velocity measurements was provided by an iodine absorption cell. Our measurements reveal that the radial velocity of 4 UMa exhibits a periodic variation of 269.3 days with a semiamplitude K=216.8m/s. A Keplerian orbit with an eccentricity, e=0.43±0.02 is the most reasonable explanation for the radial velocity variations. The orbit yields a mass function, f(m)=(2.05±0.24)x10^–7M_☉. From our high resolution spectra we calculate a metallicity of -0.25±0.05 and derive a stellar mass of 1.23M_☉±0.15 for the host star. The K giant star 4 UMa hosts a substellar companion with minimum mass Msini=7.1±1.6M_Jupiter.