A Not-so-compact Companion: Massive, Oversized White Dwarf in a Post-common-envelope Eclipsing Binary

Abstract We provide a detailed characterization of 2M07515777+1807352, a post-common-envelope eclipsing binary system with a 10.3-day, nearly (but not quite) circular orbit ( e = 0.02). This system consists of a massive white dwarf (WD; 1.08 M ⊙ ) and a 4400 K main-sequence companion (0.66 M ⊙ ). This WD is among the most massive known within post-common-envelope binary systems. We also find, through both spectral energy distribution and TESS light-curve analyses, that the WD has a radius of 1.54 ± 0.07 R ⊕ , roughly 12 σ larger than the expected value from WD mass–radius relationships. Both the Lomb–Scargle analysis and the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>v</mml:mi> <mml:mi>sin</mml:mi> <mml:mi>i</mml:mi> </mml:math> of the system indicate that the main-sequence companion is supersynchronously rotating at a period of ∼6 days, which may suggest that accretion occurred during the evolution of the system. This binary also shares similar physical characteristics with six other post-common-envelope systems hosting massive WDs, which may point to a shared formation pathway. We model the history of this system with COSMIC and find that it likely formed through an episode of common-envelope evolution following the onset of mass transfer when the progenitor primary was on either the early or thermally pulsing stages of the asymptotic giant branch. As a result of its properties, the study of 2M07515777+1807352 can provide new insights regarding many key outstanding questions in our understanding of common-envelope evolution.