The borderline system
ZZ PsA is the most uncertain of three contact-binary systems tested for signs of a terminal, unstable phase. In the paper's Darwin-instability assessment, AL Lep and ASAS J082243+1927.0 were below the standard threshold, while ZZ PsA lay close to the instability regime and could fit an unstable configuration within the reported uncertainties. The authors therefore describe ZZ PsA as a borderline case.
The work combines high-resolution VLT/UVES spectra with multi-band light curves and eclipse-timing data. The researchers used the spectra to derive radial velocities, spectroscopic mass ratios and metallicities; they fitted the light curves with a Monte Carlo framework and examined period changes through O−C timing analysis and instability criteria.
Two sets of observations, one picture
Spectroscopy produced very low mass ratios for all three systems: 0.132 ± 0.015 for AL Lep, 0.095 ± 0.018 for ASAS J082243+1927.0 and 0.078 ± 0.018 for ZZ PsA. The estimated effective temperatures were 6,285 ± 133 K, 5,880 ± 132 K and 5,898 ± 110 K, respectively. Metallicity estimates were −0.056 ± 0.030, −0.524 ± 0.038 and −0.042 ± 0.029, putting ASAS J082243+1927.0 lower than the other two on that measure.
Photometric modeling returned closely matching mass ratios: 0.1332 ± 0.0005 for AL Lep, 0.1041 ± 0.0011 for ASAS J082243+1927.0 and 0.0789 ± 0.0003 for ZZ PsA. The fitted fill-out factor, a model measure of contact depth, was 22 ± 2 percent for AL Lep, 25 ± 5 percent for ASAS J082243+1927.0 and 98 ± 2 percent for ZZ PsA. That spread describes moderate contact in the first two cases and nearly complete contact in the third.
The primary components were estimated to have masses of 1.66 ± 0.08, 1.28 ± 0.08 and 1.80 ± 0.11 solar masses for AL Lep, ASAS J082243+1927.0 and ZZ PsA, respectively. On these estimates, ZZ PsA and AL Lep have more massive primaries than ASAS J082243+1927.0.
Their orbital clocks move in different directions
The fitted period-change rates were 2.0649 ± 0.0752, −0.4398 ± 0.0428 and 0.8448 ± 0.0002 in units of 10 to the minus 7 days per year for AL Lep, ASAS J082243+1927.0 and ZZ PsA. In plain terms, the fits showed period increases for AL Lep and ZZ PsA and a decrease for ASAS J082243+1927.0. ZZ PsA also showed a cyclic modulation with a period of 11.7 ± 0.4 years.
Those timing trends were also translated into signed mass-transfer estimates: 0.39 ± 0.13, −0.07 ± 0.04 and 0.12 ± 0.07 in units of 10 to the minus 7 solar masses per year for the same three systems. The authors describe each absolute estimate as below its corresponding thermal-timescale rate, calling the exchange sub-thermal. For ASAS J082243+1927.0, they say additional angular-momentum loss or non-conservative processes may contribute, underscoring that the fitted exchange rates are model-dependent.
A comparison with single stars
To place the systems in an evolutionary frame, the researchers compared both stars in each pair with single-star reference tracks. The components did not follow those reference paths: the primaries were relatively compact and under-luminous for their mass, while the secondaries were inflated and over-luminous for a similar mass. That comparison depends on the adopted evolutionary tracks and model parameters.
The instability calculation also compared angular momentum in the stars' spins with angular momentum in their orbit. The modeled ratios were 0.113 ± 0.011 for AL Lep, 0.237 ± 0.037 for ASAS J082243+1927.0 and 0.296 ± 0.063 for ZZ PsA. The corresponding critical mass-ratio estimates were 0.055 ± 0.03, 0.088 ± 0.05 and 0.079 ± 0.05. The ZZ PsA classification remains sensitive to the uncertainty range.
A cautious evolutionary ranking
Taken together, the authors place the three binaries at different points in contact-binary evolution. They describe ASAS J082243+1927.0 as less advanced, while AL Lep and ZZ PsA, with increasing fitted periods, may be in a later contact-evolution phase. The evolutionary ranking is qualitative, and the reference tracks are not tailored to the exact systems.
The study covers only the three named systems and the specific spectroscopic, photometric and timing analyses used for them. Its evolutionary comparison is qualitative, while the instability classification for ZZ PsA depends on the reported uncertainty range.
The work is an arXiv v1 preprint dated 28 August 2026. Reduced UVES spectra are available through the ESO Phase 3 archive, while the NOA photometry and derived times of minimum are provided as ASCII tables through Zenodo and the CoBiToM project website.
Paper data and sources
Original title: CoBiToM project. III. Physical characterisation of the potential merger candidates AL Lep, ASAS J082243+1927.0, and ZZ PsA
Authors: Sofia Palafouta, Kosmas D. Gazeas, Pasi Hakala et al.
Journal/Repository: arXiv
Status: Preprint, not yet peer-reviewed
First online: 2026-08-28
DOI: Not available
Original paper · Full text