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

¹ Departament de Física, Universitat Politècnica de Catalunya, c/ Esteve Terrades 5, 08860, Castelldefels, Spain
² Grupo de Evolución Estelar y Pulsaciones, Facultad de Ciencias Astronómicas y Geofísicas, Universidad Nacional de La Plata, CONICET-IALP, Paseo del Bosque s/n, 1900, La Plata, Argentina
³ Instituto de Física y Astronomía, Universidad de Valparaíso, Av. Gran Bretaña 1111, 5030, Casilla, Valparaíso, Chile
⁴ Institut d’Estudis Espacials de Catalunya (IEEC), C/ Esteve Terradas 1, Edifici RDIT, 08860, Castelldefels, Spain

^★★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 14 August 2025
Accepted: 8 March 2026

Abstract

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.