Evidence for the emergence of stellar magnetic fields during rapid mass transfer

Binary interaction fundamentally alters the evolution of massive stars, yet the physical consequences of mass transfer remain poorly constrained. Here we present high-resolution spectroscopic observations of Plaskett's Star (HD 47129) that reveal small radial-velocity variations of its magnetic secondary, coherent with the system's 14.4 d orbital period. These measurements support an evolved, post-mass-transfer configuration in which the narrow-lined primary is a partially Roche-lobe-filling stripped star with a mass of 5.9+1.8-1.4,M, while the secondary is a 40.8+9.3-6.7,M, rapidly rotating magnetic accretor. The system provides a rare opportunity to observe the immediate aftermath of mass transfer in a massive binary and offers compelling new evidence linking binary interaction to the emergence of strong stellar magnetism. Because magnetic fields regulate angular-momentum loss and transport, their formation through mass transfer could alter the subsequent evolution of massive binaries, with consequences for stellar populations, supernova progenitors and yields, and the formation of gravitational-wave sources.