Detecting exoplanets around giant stars sheds light on the later-stage evolution of planetary systems. We observed the M giant HD 18438 and the K giant HD 158996 as part of a Search for Exoplanets around Northern circumpolar Stars (SENS) and obtained 38 and 24 spectra from 2010 to 2017 using the high-resolution Bohyunsan Observatory Echelle Spectrograph (BOES) at the 1.8m telescope of Bohyunsan Optical Astronomy Observatory in Korea. We obtained precise 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 HD 158996. We checked the chromospheric activities using Ca ii H and H lines, HIPPARCOS photometry and line bisectors to identify the origin of the observed RV variations. In the case of HD 18438, we conclude that the observed RV variations with period 719.0 days are likely to be caused by the pulsations because the periods of HIPPARCOS photometric and H EW variations for HD 18438 are similar to that of RV variations in Lomb-Scargle periodogram, and there are no correlations between bisectors and RV measurements. In the case of HD 158996, on the other hand, we did not find any similarity in the respective periodograms nor any correlation between RV variations and line bisector variations. In addition, the probability that the real rotational period can be as longer than the RV period for HD 158996 is only about 4.3%. Thus we conclude that observed RV variations with a period of 820.2 days of HD 158996 are caused by a planetary companion, which has the minimum mass of 14.0 MJup, the semi-major axis of 2.1 AU, and eccentricity of 0.13 assuming the stellar mass of 1.8 M⊙. HD 158996 is so far one of the brightest and largest stars to harbor an exoplanet candidate.

Abstract
1. INTRODUCTION
2. OBSERVATIONS AND REDUCTION
3. STELLAR MODEL
    3.1. Stellar Parameters
    3.2. Rotational Velocity and Period
4. RADIAL VELOCITY VARIATIONS
5. CHROMOSPHERIC ACTIVITY, PHOTOMETRIC ANDLINE PROFILE VARIATIONS
    5.1. Chromospheric Activity
    5.2. HIPPARCOS Photometry
    5.3. Bisector Analysis
6. RESULTS
    6.1. HD 18438
    6.2. HD 158996
7. SUMMARY AND DISCUSSION
REFERENCES

• Tae-Yang Bang(Department of Astronomy and Atmospheric Sciences, Kyungpook National University/Research and Training Team for Future Creative Astrophysicists and Cosmologists (BK21 Plus Program))
• Byeong-Cheol Lee(Korea Astronomy and Space Science Institute/Korea University of Science and Technology)
• Gwang-Hui Jeong(Korea Astronomy and Space Science Institute/Korea University of Science and Technology)
• Inwoo Han(Korea Astronomy and Space Science Institute/Korea University of Science and Technology)
• Myeong-Gu Park(Department of Astronomy and Atmospheric Sciences, Kyungpook National University/Research and Training Team for Future Creative Astrophysicists and Cosmologists (BK21 Plus Program)) Corresponding author