2026ApJ..1000....2S

Understanding the transfer of mass and angular momentum in binary interactions is crucial for modeling the evolution of any interacting binary after the first mass-transfer phase. Mass-transfer physics assumptions shape the predictions for later stages of binary evolution, such as the immediate progenitors of stripped-envelope supernovae and gravitational-wave mergers. We constrain the efficiency and stability of thermal-timescale mass transfer in massive binary evolution using the observed population of 62 massive interacting binaries on the main sequence (“Algols”) in the Milky Way and the Large and Small Magellanic Clouds. We find that purely conservative or nonconservative mass transfer cannot explain the current mass ratio and orbital period of all massive Algols. Angular momentum conservation rules out conservative mass transfer in ∼28% of massive Algols in the SMC. About three-quarters of all massive Algols are consistent with having undergone inefficient mass transfer (≲50%), while the remaining systems, mostly residing in the LMC and the Milky Way, require mass transfer to have been more efficient than 25%. For our fiducial assumption on the extent of envelope stripping, the current sample of massive Algols does not require mass transfer to be efficient at the shortest orbital periods (∼2 days) at any metallicity. We find evidence that mass transfer on the main sequence needs to be stable for initial accretor-to-donor mass ratios as unequal as ∼0.6. Unless biased by observational selection effects, the massive Algols in the SMC seem to have undergone less efficient mass transfer than those in the LMC and the Milky Way.