The double low-mass white dwarf eclipsing binary system J2102-4145 and its possible evolution

In recent years, about 150 low-mass white dwarfs (WDs), typically with masses below 0.4 M-circle dot, 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-eff,T-1 = 13 688(-72)(+65) K, log g(1) = 7.36 +/- 0.01, R-1 = 0.0211 +/- 0.0002 R-circle dot, and M-1 = 0.375 +/- 0.003 M-circle dot, while, the secondary star is characterised by T-eff,T-2 = 12 952 +/- 53 66 K, log g(2) = 7.32 +/- 0.01, R-2 = 0.0203 +/- 0.0002 0.0003 R-circle dot, and M-2 = 0.314 +/- 0.003 M-circle dot. 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 similar to 800 Myr, evolving into a helium-rich hot subdwarf star and later into a hybrid He/CO WD.