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

Context. 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. Aims. We analyse J2102–4145, an eclipsing binary composed of two low-mass He-core WDs in a 2.4-hour orbit. Methods. 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. Results. 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. Conclusions. The secondary’s unusually small radius requires an extremely thin H envelope, M H ≲ 10 −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.