The Double Low-Mass White Dwarf Eclipsing Binary System J2102-4145 and Its Possible Evolution

Approximately 150 low-mass white dwarfs, with masses below 0.4Msun, have beendiscovered. The majority of these low-mass WDs are observed in binary systemsas they cannot be formed through single-star evolution within the Hubble time.In this study, we present a comprehensive analysis of the double low-mass WDeclipsing binary system J2102-4145. Our investigation involved an extensiveobservational campaign, resulting in the acquisition of approximately 28 hoursof high-speed photometric data across multiple nights using NTT/ULTRACAM,SOAR/Goodman, and SMARTS-1m telescopes. These observations have providedcritical insights into the orbital characteristics of this system, includingparameters such as inclination and orbital period. To disentangle the binarycomponents of J2102-4145, we employed the XT GRID spectral fitting method withGMOS/Gemini-South and X-Shooter data. Additionally, we used the PHOEBE packagefor light curve analysis on NTT/ULTRACAM high-speed time-series photometry datato constrain the binary star properties. Our analysis reveals remarkablesimilarities between the two components of this binary system. For the primarystar, we determined Teff1 = 13688 +- 65 K, log g1 = 7.36 +- 0.01, R1 = 0.0211+- 0.0002 Rsun, and M1 = 0.375 +- 0.003 Msun, while the secondary star ischaracterized by Teff2 = 12952 +- 53 K, log g2 = 7.32 +- 0.01, R2 = 0.0203 +-0.0002 Rsun, and M2 = 0.31 +- 0.003 Msun. Furthermore, we observe a notablediscrepancy between Teff and R of the less massive WD compared to evolutionarysequences for WDs from the literature, which has significant implications forour understanding of WD evolution. We discuss a potential formation scenariofor this system that might explain this discrepancy and explore its futureevolution. We predict that this system will merge in about 800 Myr, evolvinginto a helium-rich hot subdwarf star and later into a hybrid He/CO WD.