An Ultra-low Mass Ratio Contact Binary: WISE J095035.2+354215

Abstract We present the first observational analysis of the totally eclipsing binary WISE J095035.2+354215 (hereafter J095035), a system whose mass ratio lies near the theoretical stability limit for binary stars. Fundamental stellar parameters of the system were determined through spectral energy distribution fitting, yielding a surface gravity of log g = 4.1 6 − 0.29 + 0.34 cm s − 2 , a metallicity of [ Fe/H ] = 0.0 5 − 0.06 + 0.08 dex , and an effective temperature of T eff = 626 2 − 34 + 43 K . Employing the PHOEBE 2.4 software package with a Markov Chain Monte Carlo sampling technique, we derived photometric parameters from multiband observations, confirming that J095035 is an overcontact binary with an ultra-low mass ratio ( q ≈ 0.064) and a contact degree of 47%. Synthesizing eclipse timing data from our observations and multiple astronomical surveys, we determined that J095035’s orbital period is experiencing a secular decline at d P d t = − 3.70 × 1 0 − 7 day yr − 1 . This secular orbital period variation is most likely driven by the joint influence of two key physical processes: mass exchange between the binary’s constituent stars and angular momentum leakage from the entire system. The sustained orbital period reduction triggers shrinkage of the critical Roche lobes, thereby enhancing the contact depth and ultimately directing the system toward coalescence into a fast-rotating merger product.