The black holes (BH) in merging BH–BH binaries are likely progeny of binary O stars. Their properties, including their spins, will be strongly influenced by the evolution of their progenitor O stars. The remarkable observation that many single O stars spin very rapidly can be explained if they accreted angular momentum from a mass-transferring, O-type or Wolf–Rayet (WR) companion before that star blew up as a supernova. To test this prediction, we have measured the spin rates of eight O stars in WR + O binaries, increasing the total sample size of such O stars’ measured spins from 2 to 10. Polarimetric and other determinations of these systems’ sin i allow us to determine an average equatorial rotation velocity from He i (He ii) lines of ve = 348 (173) km s−1 for these O stars, with individual star's ve from He i (He ii) lines ranging from 482 (237) to 290 (91) km s−1. We argue that the ∼100 per cent difference between He i and He ii speeds is due to gravity darkening. Supersynchronous spins, now observed in all 10 O stars in WR + O binaries where it has been measured, are strong observational evidence that Roche lobe overflow mass transfer from a WR progenitor companion has played a critical role in the evolution of WR + OB binaries. While theory predicts that this mass transfer rapidly spins up the O-type mass gainer to a nearly breakup rotational velocity of ve ∼ 530 km s−1, the observed average ve of the O-type stars in our sample is 65 per cent of that speed. This demonstrates that, even over the relatively short WR-phase time-scale, tidal and/or other effects causing rotational spin-down must be efficient. A challenge to tidal synchronization theory is that the two longest period binaries in our sample (with periods of 29.7 and 78.5 d) unexpectedly display supersynchronous rotation.
- binaries: close, stars: evolution, stars: massive, stars: rotation, stars: Wolf-Rayet