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The black holes (BH) in merging BH-BH binaries are likely progeny of binary Ostars. Their properties, including their spins, will be strongly influenced bythe evolution of their progenitor O stars. The remarkable observation that manysingle O stars spin very rapidly can be explained if they accreted angularmomentum from a mass-transferring, O-type or Wolf-Rayet companion before thatstar blew up as a supernova. To test this prediction, we have measured the spinrates of eight O stars in Wolf-Rayet (WR) + O binaries, increasing the totalsample size of such O stars' measured spins from two to ten. Polarimetric andother determinations of these systems' sin i allow us to determine an averageequatorial rotation velocity from HeI (HeII) lines of $v_e$ = 348 (173) km/sfor these O stars, with individual star's $v_e$ from HeI (HeII) lines rangingfrom 482 (237) to 290 (91) km/s. We argue that the $\sim$ 100\% differencebetween HeI and HeII speeds is due to gravity darkening. Super-synchronousspins, now observed in all 10 O stars in WR + O binaries where it has beenmeasured, are strong observational evidence that Roche lobe overflow masstransfer from a WR progenitor companion has played a critical role in theevolution of WR+OB binaries. While theory predicts that this mass transferrapidly spins-up the O-type mass gainer to a nearly break-up rotationalvelocity $v_e \sim 530 $ km/s, the observed average $v_e$ of the O-type starsin our sample is 65\% that speed. This demonstrates that, even over therelatively short WR-phase timescale, tidal and/or other effects causingrotational spin-down must be efficient. A challenge to tidal synchronizationtheory is that the two longest-period binaries in our sample (with periods of29.7 and 78.5 days) unexpectedly display super-synchronous rotation.

The spin rates of O stars in WR + O binaries. I. Motivation, methodology and first results from SALT