Two Ultralow-mass-ratio Binary Systems at Critical Evolutionary Stages

Abstract In this paper, we present photometric observations and period analysis of two totally eclipsing binaries with extremely low-amplitude light curves. Both binaries exhibit typical W Ursae Majoris-type light curves with total eclipses, allowing for the precise determination of the mass ratios. Light-curve solutions were derived using the Python package PHysics Of Eclipsing BinariEs in combination with a Markov Chain Monte Carlo framework. Both WISE J053525.4–200442 (hereafter J053525) and ASASSN-V J175200.35+361805.2 (hereafter J175200) are identified as shallow contact binaries with ultralow mass ratios. Specifically, J053525 exhibits a mass ratio of q ≈ 0.124 and a marginal contact degree of f ∼ 22%, while J175200 has q ≈ 0.027 and a contact degree of f ∼ 13%. Notably, J175200 represents the most extreme mass ratio observed in contact binaries to date. However, due to degeneracies between mass ratio and third-light contributions in purely photometric analyses, the derived mass ratios should be interpreted as lower limits, especially for J175200. The evolutionary states of these systems differ significantly. For J053525, a long-term period decrease is detected, with the orbital period derivative calculated as <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mfrac> <mml:mrow> <mml:mi>d</mml:mi> <mml:mi>P</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>d</mml:mi> <mml:mi>t</mml:mi> </mml:mrow> </mml:mfrac> <mml:mo>=</mml:mo> <mml:mo>−</mml:mo> <mml:mn>1.99</mml:mn> <mml:mo>×</mml:mo> <mml:mn>1</mml:mn> <mml:msup> <mml:mrow> <mml:mn>0</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>6</mml:mn> </mml:mrow> </mml:msup> <mml:mspace width="0.25em"/> <mml:msup> <mml:mrow> <mml:mspace width="0.1em"/> <mml:mtext>day yr</mml:mtext> <mml:mspace width="0.1em"/> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msup> </mml:math> . In contrast, J175200 shows a period increase: <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mfrac> <mml:mrow> <mml:mi>d</mml:mi> <mml:mi>P</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>d</mml:mi> <mml:mi>t</mml:mi> </mml:mrow> </mml:mfrac> <mml:mo>=</mml:mo> <mml:mn>1.93</mml:mn> <mml:mo>×</mml:mo> <mml:mn>1</mml:mn> <mml:msup> <mml:mrow> <mml:mn>0</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>7</mml:mn> </mml:mrow> </mml:msup> <mml:mspace width="0.25em"/> <mml:msup> <mml:mrow> <mml:mspace width="0.1em"/> <mml:mtext>day yr</mml:mtext> <mml:mspace width="0.1em"/> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msup> </mml:math> . These contrasting period trends may be indicative of ongoing mass transfer; however, alternative explanations such as third-body effects cannot be ruled out. Nevertheless, the combination of their ultralow mass ratios, shallow contact configurations, and apparent period evolution highlights their unique value in testing mass ratio theories and the thermal relaxation oscillation hypothesis.