Binary systems with red giants (binary star) are key objects for testing evolutionary models. The Algol paradox, known since the 17th century, remains a challenge: in close pairs, mass can flow from one star to another, disrupting the standard scenario. Observing both components as red giants in a semi-detached configuration, with one star filling its Roche limit, had not been achieved until now. Such systems are direct precursors to the common envelope stage, which determines whether the stars merge or become a pair of white dwarf stars. Thus, the discovery of J05+50 is a rare chance to glimpse the future of close binary systems.
Medium-resolution spectra from the LAMOST survey covered 41 epochs, enabling the spectral disentangling method to separate the contributions of both stars and measure their radial velocities. Spectroscopy provided temperatures and surface gravity, while photometry from ZTF and WISE revealed sinusoidal brightness variations without eclipses—a sign of ellipsoidal distortion. The system parameters were refined using PHOEBE, and the evolution was reconstructed with the MESA code, applying numerical simulation. The loss of angular momentum via quadrupole radiation formula according to the Lev Landau–Lifshitz formula and non-conservative mass transfer were taken into account.
The best model gives an orbital period of 59.96 days, mass ratio q = 0.125, primary mass about 8.5 M⊙, radii 52 and 30 R⊙, orbital inclination ~45°. Both stars are red giants with temperatures ~5000 K and metallicity [Fe/H] ≈ –0.13. The light curve is precisely described by a pure sine wave with an amplitude of ~0.02 mag, typical for a non-eclipsing system. In the dynamic spectrum of the hydrogen Hα line, an emission component blueshifted is detected—evidence of recent outflow of material moving toward us, and a possible Accretion disk. Evolutionary modeling showed that the system started as a close pair with a period of 2.24 days and masses of 5.13 and 4.21 M⊙; through mass exchange the orbit expanded to its current size, and in less than 0.1 million years the components will fill a common Roche lobe and merge.
The discovery of the first system of two red giants in the active mass-transfer phase confirms the theory of stellar evolution in close pairs. Such objects are the missing link between wide binaries with red giants and merging systems undergoing the common envelope stage. The measured parameters allow calibration of mass and angular momentum exchange models, critical for understanding the population of double white dwarfs and future observations of gravitational waves.
Accumulation of precise photometric data, especially from the Gaia mission, will allow measuring the period change and testing the model's prediction of its increase. High-resolution spectroscopy during the Hα emission phase will provide a detailed picture of the outflowing material. Searching for similar systems in large surveys (SDSS-V, WEAVE) will assess the frequency of such phases and refine merger scenarios.
The results are important for stellar evolution theory, close binary physics, predicting the population of double white dwarfs, and planning the search for gravitational wave sources with space-based interferometers.
High-precision photometry to search for shallow eclipses and detect period changes, along with spectroscopic monitoring of emission lines to map material flows.
The system is directly connected to the problem of common envelope evolution—a key unsolved issue in close binary physics, affecting the formation of short-period double white dwarfs and the population of gravitational wave sources.
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What the work does
This work reports the discovery of the close binary system J05+50, consisting of two red giants in a semi-detached configuration with active mass transfer. Based on multi-epoch spectroscopy from LAMOST and photometry from ZTF/WISE, the authors disentangled the component spectra, measured radial velocities, and built a light curve model in PHOEBE. They then reconstructed the system's evolution in MESA, accounting for non-conservative mass exchange and angular momentum loss.
Where its strength is
For the first time, the existence of a binary system of two red giants in the mass-exchange phase—a direct precursor to the common envelope stage—has been observationally confirmed. The derived orbital and stellar parameters offer a rare opportunity to calibrate mass transfer models at a short, dynamically important stage.
Where it could go next
Worth trying
Why this may matter
Such systems are the missing link between wide pairs with red giants and merging objects. Studying them directly impacts predictions of the merger rate of double white dwarfs and, consequently, the expected gravitational wave signals.
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The Algol paradox is named after the star Algol (β Persei), whose variability was noticed as early as 1667 by astronomer Geminiano Montanari, and first explained by John Goodricke in 1782—long before the discovery of X-ray binaries and accretion disks.