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 companion before that star blew up as a supernova .
To test this prediction , we have measured the spin rates of eight O stars in Wolf-Rayet ( WR ) + O binaries , increasing the total sample size of such O stars ’ measured spins from two to ten .
Polarimetric and other determinations of these systems ’ sin i allow us to determine an average equatorial rotation velocity from HeI ( HeII ) lines of v _ { e } = 348 ( 173 ) km/s for these O stars , with individual star ’ s v _ { e } from HeI ( HeII ) lines ranging from 482 ( 237 ) to 290 ( 91 ) km/s .
We argue that the \sim 100 % difference between HeI and HeII speeds is due to gravity darkening .
Super-synchronous 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 break-up rotational velocity v _ { e } \sim 530 km/s , the observed average v _ { e } of the O-type stars in our sample is 65 % that speed .
This demonstrates that , even over the relatively short WR-phase timescale , 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 days ) unexpectedly display super-synchronous rotation .