Aims. To search for extrasolar planets around intermediate-mass stars, we are conducting a precise RV survey around K-giants. Methods. We present high-accuracy RV measurements of {alpha} Ari made from November 2003 to February 2010. This star belongs to our sample of 55 K-giants studied for extrasolar planet and pulsation searches using the fiber-fed Bohyunsan Observatory Echelle Spectrograph (BOES) attached to the 1.8-m telescope at Bohyunsan Optical Astronomy Observatory (BOAO) in Korea. Results. We find a planetary companion with long-period and low-amplitude radial velocity (RV) variations in oscillating K2 III star {alpha} Ari (HD 12929). We do not find the correlation between RV variations and chromospheric activity indicators (Ca II H & K region, H{alpha} line). The bisector analysis also shows that the bisector velocity span (BVS) does not show any obvious correlation with RV variation but has periodic component that may be attributed to the rotation of the star. If the RV variation is indeed caused by a planetary companion, an orbital solution with a period of P = 381 days, a semi-amplitude of K = 41 m/s, and an eccentricity of e = 0.25 fits the data best. Assuming a possible stellar mass of M_star = 1.5 M_sun, we estimate the minimum mass for the planetary companion of m2 sin i = 1.8 M_Jupiter with an orbital semi-major axis of 1.2 AU. Conclusions. Our finding of a likely exoplanet in {alpha} Ari supports searching for extrasolar planets around giant stars with multiperiodic oscillations.
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