Revealing Optically Hidden Companions to Intermediate-Mass Stripped Stars with Radial Velocities

The recent discovery of a population of intermediate-mass helium stars (about 2-8 M) in the Magellanic Clouds has opened a new observational window into massive binary evolution. These hot, helium-rich stars are thought to be products of binary interaction, and some show no evidence for a luminous companion in their optical spectra. Here, we present multi-epoch radial velocity (RV) measurements for eight of these systems obtained with Magellan/MagE from 2018-2023. Five helium stars exhibit significant RV variability consistent with binary motion, while three do not. The five helium stars with evidence of binarity have orbital periods from 16 hours to 300 days, and eccentricities consistent with zero, implying companion mass lower limits from about 0.6-2.9 M. We discuss implications of their orbits for the nature of the unseen companions, and find that low-mass late-pre/early-main sequence stars (1.5-3.1 M), low-mass stripped stars and compact objects are currently viable. We identify an unexpected correlation between surface hydrogen abundance and binarity within this sample: helium stars in binaries are hydrogen-poor (X H,surf about 0.3-0.4), whereas helium stars without detected binary motion are nearly hydrogen-free (X H,surf 0.05). This bimodality suggests that low RV variability in a subset of the sample is not simply due to inclination, but rather a distinct evolutionary pathway. We consider three possible explanations for these objects: wide binaries, disrupted binaries, and stellar mergers. The binary properties of this first sample of intermediate-mass helium stars is diverse, and provides important empirical constraints for binary evolution and compact object formation.