Optical flares detected on a contact binary: the first photometric and spectroscopic analysis of a long-period low mass ratio contact binary HAT 307-0007476

This paper presents the photometric and spectroscopic analysis of a long-period totally eclipsing contact binary (HAT 307-0007476) for the first time. This system is a low mass ratio (q~ 0.114) and medium contact binary (f~ 37.1~\ per\ cent). Two flare events were detected in multiple bands observations in December 2022. The interval between the two flare events is 4 d. The average duration of these two flares is about 2289 s. Both the two flares achieve the energy levels of superflares. The excess emission of the H_α line in the LAMOST spectra of this object was analysed, indicating its chromospheric activity. The O–C diagram showed a long-term orbital period increase, which is due to the mass transfer between the two component stars. We conclude that HAT 307-0007476 is currently in a stable region based on both Jspin/Jorb and the comparison between the instability parameters and its current values.