Extreme mass loss during common envelope evolution: The origin of the double low-mass white dwarf system J2102--4145

Eclipsing close double white dwarf (WD) systems, by providing precise radii and masses, offer a unique opportunity to directly constrain hydrogen-envelope (M_ H) retention and test common-envelope (CE) evolution in low-mass stars. We analyse J2102--4145, an eclipsing binary composed of two low-mass He-core WDs in a 2.4-hour orbit. By comparing the observed radii and effective temperatures with updated CE and stable Roche-lobe overflow (SRLOF) models, we confirm that both stars are He-core WDs. The primary (0.375,M_⊙) is consistent with SRLOF models that retain thick H envelopes and sustain residual burning, while the secondary (0.314,M_⊙) can only be reproduced by CE models with nearly complete envelope removal. The cooling ages---∼220,Myr for the secondary and ∼260–510,Myr for the primary, depending on the residual nuclear contribution--- support a formation sequence in which the primary formed first via SRLOF, followed by a CE phase producing the compact secondary. Energy-budget reconstruction of the CE yields progenitor and orbital parameters consistent with this picture. The secondary’s unusually small radius requires an extremely thin H envelope, M_ ̊m H łesssim10^ -7 ,M_⊙, well below the predictions of standard bifurcation criteria. J2102--4145 thus provides one of the strongest observational constraints on post-CE M_ H in low-mass WDs and a benchmark challenge to current prescriptions of envelope ejection.