Letter to the Editor

Detection of the lowest mass ratio contact binary in the universe: TYC 3801-1529-1

¹ Shandong Key Laboratory of Optical Astronomy and Solar-Terrestrial Environment, School of Space Science and Technology, Institute of Space Sciences, Shandong University, Weihai, Shandong 264209, China
² Yunnan Observatories, Chinese Academy of Sciences, Kunming 650216, China
³ Department of Astronomy and Space Science, Chungbuk National University, Cheongju 361-763, Korea

^⋆ Corresponding author; kaili@sdu.edu.cn

Received: 21 August 2024
Accepted: 9 November 2024

Abstract

This paper presents the first analysis of the contact binary TYC 3801-1529-1. We observed four sets of multiple bands complete light curves and one set of radial velocity curve of the primary component. Based on a simultaneous investigation of our observed and TESS light curves and the radial velocity curve, we found that TYC 3801-1529-1 is an extremely low-mass-ratio, medium contact binary with q = 0.0356, with the contribution of the third light at a level of about 10%. Its mass ratio is lower than V1187 Her, making TYC 3801-1529-1 the lowest mass-ratio contact binary ever found in the universe. The light curves observed in 2022 are asymmetric, which is aptly explained by a hot spot on the primary component. A 16-year eclipse timings analysis indicates a secular increase orbital period with a rate of dp/dt = 7.96(±0.35)×10⁻⁷ d yr⁻¹. We studied the stability of this target and identified that not only the value of J_spin/J_orb, but also the mass ratio surpass the unstable boundary. Hence, TYC 3801-1529-1 presents a challenge to theoretical research and ought to be considered a progenitor of a contact binary merger.