Determine how conservative Case A mass transfer is

Determine how conservative Case A mass transfer is in close massive binaries, including the fraction of transferred mass accreted by the companion star.

Background

Close-binary stable mass transfer can substantially alter the accretor’s structure, orbital evolution, and the masses of the eventual black holes. Although observed counterparts suggest considerable accretion during the first mass-transfer phase, the fraction of transferred material retained by the accretor remains unresolved.

References

Our model is unable to reproduce the extreme ($q\sim 7$) mass ratio implied by the observations while simultaneously reproducing the orbital and physical properties of the system.

— Evidence for the emergence of stellar magnetic fields during rapid mass transfer  (2609.25526 - Wade et al., 22 Sep 2026) in Section 6.1, “Summary”; Section 7, “MESA simulation setup”; Section 8, “On the discrepancy between the observed mass ratio and that predicted by evolutionary modeling”

Mass transfer efficiency remains as a significant uncertainty in binary evolution models, and particularly, the evolution of a mass transferring system after the accretor reaches critical rotation is uncertain (see discussions in and ).

— Evidence for the emergence of stellar magnetic fields during rapid mass transfer  (2609.25526 - Wade et al., 22 Sep 2026) in Section 7, “MESA simulation setup”; Section 8, “On the discrepancy between the observed mass ratio and that predicted by evolutionary modeling”

Observational counterparts of Case\,A systems have been identified at many key evolutionary stages, which are Algol systems, close WR star binaries, close BH+O binaries, WR star X-ray binaries \citep[][and references therein]{Xu2025arXiv251220054X}, suggesting that considerable mass accretion has occurred during the first MT phase, although how conservative it is remains unclear \citep{Petrovic2005,Sen2022,Nuijten2025A&A...695A.117N,Sen2026ApJ..1000....2S}.

— Can isolated binaries form unequal-mass binary black-hole mergers with a high-spin primary black hole?  (2609.05128 - Xu et al., 4 Sep 2026) in Section 2.2, subsection “Close binaries: stable mass transfer”