Unveiling a Pulsating Pair: A Close Binary with a Pre-White Dwarf in WASP 1021-28
The study by Lee et al. (2025) focuses on a fascinating binary star system called WASP 1021-28, where one of the stars is a nearly-formed white dwarf. This type of system belongs to a class called EL CVn binaries, which are made up of a normal A- or F-type star (like our Sun but hotter and more massive) and a faint, small companion that is on its way to becoming a white dwarf. These binaries are the product of mass transfer — where one star loses material to the other — and offer a chance to understand both stellar evolution and how stars interact when they live so close together.
Observing Eclipses with TESS
The authors used data from two main sources to study WASP 1021-28. One was the TESS space telescope, which monitored the system’s brightness over time and captured regular eclipses as the stars passed in front of each other. These light curves, observed over four separate periods, allowed the team to measure how long the stars take to orbit each other — just under a day. They also used the Very Large Telescope (VLT) to take high-resolution spectra of the system, which helped them determine how fast each star moves. By looking at shifts in the absorption lines of each star, they could estimate their velocities and confirm that both stars' lines could be traced — something not always easy when one star is very faint.
Measuring the Stars’ Properties
Using both sets of data, the researchers built a detailed model of the system. This involved fitting the light and velocity curves to estimate the masses, sizes, and temperatures of both stars. The primary (brighter) star is about 1.6 times the mass of the Sun and has a radius of 1.75 times that of the Sun, while the secondary — a hot, low-mass star that hasn’t yet become a white dwarf — has only 0.19 solar masses and is about a third of the Sun’s size. They also found evidence for a third, more distant object in the system — likely a K-type star discovered in earlier infrared imaging — contributing a small amount of light (about 2.9%) to the total.
Pulsations in the Binary System
Beyond the basic properties, Lee et al. explored how the stars in WASP 1021-28 pulse. These pulsations, or small brightness variations, can tell us about what's happening deep inside the stars. By carefully removing the regular changes caused by eclipses and orbital motion, they found signs of two types of pulsations. A slow, low-frequency oscillation (at 1.32 times per day) seems to originate from the primary star and is likely a γ Doradus-type pulsation, a kind of variability often found in A-type stars. The faster, high-frequency pulsations — some over 130 times per day — appear to come from the faint secondary star and match those expected from extremely low-mass (pre-He) white dwarf pulsators.
Comparisons and Modeling Choices
The researchers compared their findings to earlier work and pointed out some differences, especially in how they measured the velocities and how much light comes from the suspected third star. Their rotational function approach gave slightly different but more precise results than other methods. Their model also fits well with evolutionary tracks for pre-white dwarfs, indicating that the faint companion star is at a transitional point on its way to becoming a true white dwarf.
Placement in the Galaxy
Lastly, the study places WASP 1021-28 within our Galaxy's thin disk — the region where most stars like our Sun are found. Its space motion and chemical composition both point to this population. The system’s compact orbit and the presence of a distant third star suggest a complex history, likely shaped by interactions with that outer companion. All in all, WASP 1021-28 is a stellar laboratory for studying how close binaries evolve, how stars pulsate, and how interactions in multiple-star systems shape their destinies.
Source: Lee
