A family of binaries with an extreme mass ratio

¹ Groupe d’Astrophysique des Hautes Energies, STAR, Université de Liège, B5c, Allée du 6 Août 19c, 4000 Sart Tilman, Liège, Belgium
² Astronomical Institute, Department of Physics and Astronomy, University of Wroclaw, Kopernika 11, 51-622 Wroclaw, Poland
³ Royal Observatory of Belgium, Avenue Circulaire/Ringlaan 3, 1180 Brussels, Belgium
⁴ LIRA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Université Paris Cité, CY Cergy Paris Université, 92190 Meudon, France
⁵ DTU Space, Technical University of Denmark, Elektrovej 327, 2800 Kgs., Lyngby, Denmark

^★ Corresponding author: ynaze@uliege.be

Received: 16 July 2025
Accepted: 1 October 2025

Abstract

Context. Multiplicity is ubiquitous among massive stars. While the stellar components usually display similar masses, some binaries with extremely low mass ratios were also observed. Some of them are primordial, while others arise from binary interactions. The identification of systems with extreme mass ratios brings valuable information, notably on the origin of fast rotation in massive stars.

Aims. We identify new short-period systems with extreme mass ratios through the detection of eclipses and reflection effects.

Methods. The physical properties of a dozen newly identified cases were precisely evaluated through high-quality photometry and spectroscopy.

Results. In addition to characterizing these binaries, we found a clear signature of apsidal motion in one system, and three other systems display long-term shifts in eclipse times.

Conclusions. All systems we reported here are composed of a massive star and a cool low-mass companion. They are therefore primordial cases. This doubles the known number of these systems in the Galaxy. In this context, it is important to note that most massive stars in these systems, as well as in previous systems reported in the literature, rotate fast (supersynchronous compared to the orbital motion). The high incidence of fast rotation in these nascent binaries provides strong constraints for star formation models.