2024A&A...685A...9A

In recent years, about 150 low-mass white dwarfs (WDs), typically with masses below 0.4 M_⊙, have been discovered. The majority of these low-mass WDs are observed in binary systems as they cannot be formed through single-star evolution within Hubble time. In this work, we present a comprehensive analysis of the double low-mass WD eclipsing binary system J2102-4145. Our investigation encompasses an extensive observational campaign, resulting in the acquisition of approximately 28 h of high-speed photometric data across multiple nights using NTT/ULTRACAM, SOAR/Goodman, and SMARTS-1m telescopes. These observations have provided critical insights into the orbital characteristics of this system, including parameters such as inclination and orbital period. To disentangle the binary components of J2102-4145, we employed the XTGRID spectral fitting method with GMOS/Gemini-South and X-shooter data. Additionally, we used the PHOEBE package for light curve analysis on NTT/ULTRACAM high-speed time-series photometry data to constrain the binary star properties. Our analysis unveils remarkable similarities between the two components of this binary system. For the primary star, we determine $ T_{\mathrm{eff,1}} = 13\,688^{+65}_{-72} $ K, log g₁ = 7.36 ± 0.01, R₁ = 0.0211 ± 0.0002 R_⊙, and M₁ = 0.375 ± 0.003 M_⊙, while, the secondary star is characterised by $ T_{\mathrm{eff,2}} = 12\,952^{+53}_{-66} $ K, log g₂ = 7.32 ± 0.01, $ R_2=0.0203^{+0.0002}_{-0.0003}\,R_\odot $, and M₂ = 0.314 ± 0.003 M_⊙. Furthermore, we found a notable discrepancy between T_eff and R of the less massive WD, compared to evolutionary sequences for WDs from the literature, which has significant implications for our understanding of WD evolution. We discuss a potential formation scenario for this system which might explain this discrepancy and explore its future evolution. We predict that this system will merge in ∼800 Myr, evolving into a helium-rich hot subdwarf star and later into a hybrid He/CO WD.