2026ApJ...998...55Z

Theoretical investigations predict a minimum mass ratio of about 0.085-0.095 for contact binaries. If the mass ratio is below the critical value, a tidal instability occurs, forcing the stars to merge into a single object. Observationally, however, more and more W UMa-type binaries have been found to have mass ratios lower than the theoretical limit. In this study, we try to solve this discrepancy by considering the stellar structure and evolution probed through asteroseismology. We searched for and detected gravity (g)- and Rossby (r)-mode pulsations in four contact binaries with mass ratios less than 0.1. A seismic study of the pulsations led us to identify the pulsation modes and determine the near-core and envelope rotation rates as well as the asymptotic period spacings of the pulsating, primary stars. By asteroseismic modeling, the stellar parameters were constrained, and the structure models were constructed. This enables us to determine the gyration radii of the primaries and investigate in detail the orbital stability of the contact systems. We indicate that both the mass and age have independent effects on orbital stability, with massive and older primaries being more stable. Moreover, differential core-to-surface rotation would significantly reduce the gyration radius and hence the minimum mass ratio of a contact system. Adopting the median value of the gyration radius derived for the four stars, we estimate a critical mass ratio of 0.042-0.044, depending on the filling factor.