It is difficult to distinguish the pure signal produced by an orbiting planetary companion around giant stars from other possible sources, such as stellar spots, pulsations, or certain activities. Since 2003, we have obtained radial (RV) data from evolved stars using the high-resolution, fiber-fed Bohyunsan Observatory Echelle Spectrograph (BOES) at the Bohyunsan Optical Astronomy Observatory (BOAO). Here, we report the results of RV variations in the binary star HD 135438. We found two significant periods: 494.98 d with eccentricity of 0.23 and 8494.1 d with eccentricity of 0.83. Considering orbital stability, it is impossible to have two companions in such close orbits with high eccentricity. To determine the nature of the changes in the RV variability, we analyzed indicators of stellar spot and stellar chromospheric activity to find that there are no signals related to the significant period of 494.98 d. However, we calculated the upper limits of rotation period of the rotational velocity and found this to be 478–536 d. One possible interpretation is that this may be closely related to the rotational modulation of an orbital inclination at 67–90 degrees. The other signal corresponding to the period of 8494.1 d is probably associated with a stellar companion orbiting the giant star. A Markov Chain Monte Carlo (MCMC) simulation considering a single companion indicates that HD 135438 system hosts a stellar companion with 0.57+0.017 −0.017 M⊙ with an orbital period of 8498 d.

Introduction
Observations and Data Reduction
Stellar Characteristics
    Fundamental Parameters
    Stellar Properties
Orbital Solutions
    Orbital Stability
Origin of the RV Variations
    Rotation Modulations by Spot Indicators
    Rotation Modulations by Chromospheric Activity Indicators
    Pulsation
    Rotation Period
Discussion
Acknowledgments
References
RV Measurements
Corner Plot

• Byeong-Cheol Lee(Korea Astronomy and Space Science Institute, Daejeon 34055, Republic of Korea, Korea University of Science and Technology, Daejeon 34113, Republic of Korea) Corresponding author
• Jae-Rim Koo(Kongju National University, Gongju-si, Chungcheongnam-do 32588, Republic of Korea)
• Yeon-Ho Choi(Korea Astronomy and Space Science Institute, Daejeon 34055, Republic of Korea, Korea University of Science and Technology, Daejeon 34113, Republic of Korea)
• Tae-Yang Bang(Department of Astronomy and Atmospheric Sciences, Kyungpook National University, Daegu 41566, Republic of Korea)
• Beomdu Lim(Kongju National University, Gongju-si, Chungcheongnam-do 32588, Republic of Korea)
• Myeong-Gu Park(Department of Astronomy and Atmospheric Sciences, Kyungpook National University, Daegu 41566, Republic of Korea)
• Gwanghui Jeong(Space Science Research Center, Antbridge Inc, Daejeon 34120, Republic of Korea)