First Photometric, Orbital Period, and Tidal Instability Investigations for Four Contact Binaries with Extremely Low Mass Ratios

Contact binaries with extremely low mass ratios, known as the most plausible progenitors of luminous red novae, are critical targets for refining theoretical models of binary mergers and investigating the lower mass ratio limit of contact binaries. In this work, we present the first photometric, orbital period, and tidal instability analyses for four total-eclipse contact binaries with low-amplitude light curves. Multiband photometric solutions confirm that all four targets are contact binaries with mass ratios q < 0.1. The asymmetric light curves for three systems are well reproduced by either a cool or hot spot on the primary component. For two binaries, significant third lights are detected, suggesting that they may be a hierarchical triple system. Orbital period analyses reveal that all four binaries exhibit cyclic period variations, which can be attributed to either the light-time effect induced by a distant third body or cyclic magnetic activity of their primary components. We also compiled a catalog of 177 extremely low mass ratio contact binaries ( q ≤ 0.15). Accounting for both metallicity and distortion effects, we derived the critical mass ratio for tidal instability and analyzed the stability of these systems. Our results demonstrate that low metallicity significantly reduces the critical mass ratio for tidal instability. However, there are still nine systems, including our one target, that remain tidally unstable. Finally, we discuss potential factors influencing the binary merger criterion.