TESS light curves and period changes in low-mass eclipsing binary BB Persei

We present a detailed analysis of the low-mass detached eclipsing binary system BB Persei, which contains two K-type stars in a circular orbit with a short period of 0.4856 d. We used light curves from the Transiting Exoplanet Survey Satellite (Tess), which observed BB Per in five sectors, to determine its photometric properties and a precise orbital ephemeris. The solution of the Tess light curve in PHOEBE results in a detached configuration, where the temperature of the primary component was fixed to T1 = 5 300 K according to LAMOsT, which gives us T2 = 5050 +/- 50 K for the secondary. The spectral type of the primary component was derived as K0 and the photometric mass ratio was estimated q = 0.90. Slow period changes on the current O-C diagram spanning the past 25 years indicate the presence of a third body orbiting the eclipsing pair with an orbital period of about 22 years. The companion could be a red dwarf of spectral type M6-M7 with a minimal mass of about 0.1 M ae. The characteristics and temporal variation of the dark region on the surface of the secondary component were estimated.