SHI Xiangdong, Prof. QIAN Shengbang and their colleagues from Yunnan Observatories of the Chinese Academy of Sciences, revealed that HL Draconis (HL Dra) is an oscillating eclipsing binary of Algol type (oEA) whose secondary component is almost filling its critical Roche lobe, and found that the primary component oscillates in different pulsating modes and the secondary component has spots and flares and a cycle of magnetic activities. The study was published in Monthly Notices of the Royal Astronomical Society.

Binary systems are the most reliable object to derive absolute stellar parameters and to create stellar evolutionary models. If a pulsator is a component of binary system, it can be studied by the method used to analyze the binary systems. oEA is an Algol type binary system in which the primary component shows δ Sct-like oscillations.

By analyzing the light curves observed by the Transiting Exoplanet Survey Satellite (TESS), the researchers have detected that HL Dra is Algol binary system with a mass ratio of 0.361 where the radius filling factor of the primary and secondary component is 87% and 98%, respectively. The secondary component is almost filling its critical Roche lobe, but not entirely.

During TESS observation, both the variable O’Connell effect and the existence of superflares indicated that the secondary of HL Dra should have strong magnetic activities. The researchers collected the sky survey data of more than 100 years, and discovered that the O-C curve shows a sinusoidal variation with a period of 129.88 years, which might be caused by a cool red dwarf third body.

After the removal of the binary model, the researchers detected a total of 252 pulsation frequencies from the TESS light curves, including 28 multiplets of tidally split frequencies. They identify two radial modes, 21 non-radial p-modes and three non-radial f-modes from the possible independent frequencies.

All of these features revealed that HL Dra is a critical system for further investigations of binary formation and evolutions and for tidal interactions on stellar pulsations and on magnetic activities.