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Detecting exoplanets around giant stars sheds light on the later-stageevolution of planetary systems. We observed the M giant HD 18438 and the Kgiant HD 158996 as part of a Search for Exoplanets around Northern circumpolarStars (SENS) and obtained 38 and 24 spectra from 2010 to 2017 using thehigh-resolution Bohyunsan Observatory Echelle Spectrograph (BOES) at the 1.8mtelescope of Bohyunsan Optical Astronomy Observatory in Korea. We obtainedprecise RV measurements from the spectra and found long-period radial velocity(RV) variations with period 719.0 days for HD 18438 and 820.2 days for HD158996. We checked the chromospheric activities using Ca ii H and H_alphalines, HIPPARCOS photometry and line bisectors to identify the origin of theobserved RV variations. In the case of HD 18438, we conclude that the observedRV variations with period 719.0 days are likely to be caused by the pulsationsbecause the periods of HIPPARCOS photometric and H alpha EW variations for HD18438 are similar to that of RV variations in Lomb-Scargle periodogram, andthere are no correlations between bisectors and RV measurements. In the case ofHD 158996, on the other hand, we did not find any similarity in the respectiveperiodograms nor any correlation between RV variations and line bisectorvariations. In addition, the probability that the real rotational period can beas longer than the RV period for HD 158996 is only about 4.3%. Thus we concludethat observed RV variations with a period of 820.2 days of HD 158996 are causedby a planetary companion, which has the minimum mass of 14.0 MJup, thesemi-major axis of 2.1 AU, and eccentricity of 0.13 assuming the stellar massof 1.8 M_sun. HD 158996 is so far one of the brightest and largest stars toharbor an exoplanet candidate.

Search for exoplanets around northern circumpolar stars III. long-period radial velocity variations in hd 18438 and hd 158996