Nature of HD 251108: an RS CVn binary with a long-term evolving spot

Recently, the Lobster Eye Imager for Astronomy (LEIA) detected the longest-lasting and most energetic stellar X-ray flare event from HD 251108. In this work, we re-determined the atmospheric parameters of HD 251108 using three spectroscopic observations obtained with the 2.4 m Lijiang Telescope. Combined with the stellar radius derived from spectral energy distribution fitting, we found that HD 251108 contains a K-type giant with a mass of approximately 1.3 $M_{\odot}$. Long-term photometric monitoring over 12 years reveals a modulation suggestive of a stellar activity cycle, but inconclusive given the limited time span to date. Light curve fitting indicates that the variations in both amplitude and shape are primarily driven by the evolution of a large spot. The fitting further indicates that the spot migrated from low latitudes toward the pole between 2014 and 2020, and began to recede from the pole after 2022. Using spot parameters from light curve fitting, we found that the observed radial velocity variations arise from both the spot-induced distortions and the Keplerian orbital motion of the giant star. Additionally, we detect a possible M-dwarf companion with a mass of approximately 0.25 $M_{\odot}$. Our finding suggests a notable effect on the radial velocity caused by stellar magnetic activity.