WISE J185503.7+592234: A particularly low mass ratio contact binary approaching merger

In this paper, the total eclipsing binary WISE J185503.7+592234 (hereinafter J185503), whose mass ratio is around the mass ratio limit, was observed and investigated for the first time. The stellar atmospheric parameters of the primary (more massive) component were calculated by the spectral fitting, and the following results are obtained: T eff  = 5747 ± 25 K, [Fe/H] = −0.16 ± 0.04 dex, logg = 4.78 ± 0.05 cm s ⁻² . Although J185503 has a very small amplitude (∼ 0.17 mag in V band) and a short period (0.2800782 d), its light curves exhibit total eclipses (eclipse duration: ∼ 40 min, which is about 0.1 phase). The light curves derived from TESS exhibit some unusual behaviour, such as inversion of the O’Connell effect, exchange of occultation, and transit depths. Photometric solutions obtained from the multiband observations suggest that J185503 has a particularly low mass ratio (q = 0.0514) and a moderate contact degree of $26{{\ \rm per\ cent}}$ . Based on 103 newly determined times of minimum spanning 14 yr, a secular period decrease was found, which is calculated to be dP/dt = −2.24 × 10 ⁻⁷ d yr ⁻¹ . Through analysing the instability criterion, such as the instability separation, the instability mass ratio, and the ratio of spin angular momentum (J spin ) to the orbital angular momentum (J orb ), J185503 can be regarded as a progenitor of luminous red novae.