Detection of an exoplanet around the evolved K giant HD 66141

Aims. We have been carrying out a precise radial velocity (RV) survey for K giants to search for and study the origin of the low-amplitude and long-periodic RV variations. Methods. We present high-resolution RV measurements of the K2 giant HD 66141 from December 2003 to January 2011 using the fiber-fed Bohyunsan Observatory Echelle Spectrograph (BOES) at the Bohyunsan Optical Astronomy Observatory (BOAO). Results. We find that the RV measurements for HD 66141 exhibit a periodic variation of 480.5 +/- 0.5 days with a semi-amplitude of 146.2 +/- 2.7 m s(-1). The HIPPARCOS photometry and bisector velocity span (BVS) do not show any obvious correlations with RV variations. We find indeed 706.4 +/- 35.0 day variations in equivalent width (EW) measurements of H-alpha line and 703.0 +/- 39.4 day variations in a space-born measurements 1.25 mu flux of HD 66141 measured during the COBE/DIRBE experiment. We reveal that a mean value of long-period variations is about 705 +/- 53 days, whose origin is a rotation period of the star and variability that is caused by surface inhomogeneities. For the 480-day periods of RV variations an orbital motion is the most likely explanation. Assuming a stellar mass of 1.1 +/- 0.1 M-circle dot for HD 66141, we obtain a minimum mass for the planetary companion of 6.0 +/- 0.3 M-Jup with an orbital semi-major axis of 1.2 +/- 0.1 AU and an eccentricity of 0.07 +/- 0.03.