HD 26172: an active solar-type subgiant in a close binary system

ABSTRACT We present the first comprehensive photometric and spectroscopic analysis of the RS CVn system HD 26172, robustly determining the previously debated evolutionary state of its primary star. Since this system is a single-lined spectroscopic binary with spot-induced light curve modulations, we derived its physical parameters by combining the TESS light curves, the radial velocity curve from our observations, and the primary-star mass estimates based on three complementary methods.Our results reveal that HD 26172 is a detached binary system composed of a $1.25 \pm 0.32\, {\rm M}_{\odot }$ subgiant and a $0.63 \pm 0.11\, {\rm M}_{\odot }$ main-sequence star. The conclusion of subgiant primary is also supported by the absence of lithium absorption and no observed infrared excess. Using long-term photometry from the KWS survey, we detected a tentative stellar activity cycle of 5635 d with an amplitude of 0.04 mag in HD 26172. Additionally, we identified ten optical flare events exhibiting temporally clustered outburst behaviour. The presence of a long-term activity cycle, pronounced starspot activity, and frequent optical flares makes HD 26172 a valuable laboratory for studying magnetic activity in subgiants within close binary systems.