Context : 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 Thüringer Landessternwarte Tautenburg .

Aims : To search for sub-stellar companions to giant stars and to understand the nature of the diverse radial velocity variations exhibited by K giant stars .

Methods : We present precise stellar radial velocity measurements of the K1III giant star 4 UMa ( HD 73108 ) .
These were obtained using the coudé echelle spectrograph of 2-m Alfred Jensch Telescope .
The wavelength reference for the radial velocity measurements was provided by an iodine absorption cell .

Results : Our measurements reveal that the radial velocity of 4 UMa exhibits a periodic variation of 269.3 days with a semiamplitude K = 216.8 m s ^ { -1 } .
A Keplerian orbit with an eccentricity , e = 0.43 \pm 0.02 is the most reasonable explanation for the radial velocity variations .
The orbit yields a mass function , f ( m ) = ( 2.05 \pm 0.24 ) \times 10 ^ { -7 } M _ { \odot } .
From our high resolution spectra we calculate a metallicity of - 0.25 \pm 0.05 and derive a stellar mass of 1.23 M _ { \odot } \pm 0.15 for the host star .

Conclusions : The K giant star 4 UMa hosts a substellar companion with minimum mass M sin i = 7.1 \pm 1.6 M _ { Jupiter } .