  Some massive, merging black holes (BH) may be descendants of binary O stars.
The evolution and mass transfer between these O stars determines the spins of
their progeny BH. These will be measurable with future gravitational wave
detectors, incentivizing the measurement of the spins of O stars in binaries.
We previously measured the spins of O stars in Galactic Wolf-Rayet (WR) + O
binaries. Here we measure the vsini of four LMC and two SMC O stars in WR + O
binaries to determine whether lower metallicity might affect the spin rates. We
find that the O stars in Galactic and Magellanic WR + O binaries display
average vsini = 258 +/- 18 km/s and 270 +/- 15 km/s, respectively. Two LMC O
stars measured on successive nights show significant line width variability,
possibly due to differing orbital phases exhibiting different parts of the O
stars illuminated differently by their WR companions. Despite this variability,
the vsini are highly super-synchronous but distinctly subcritical for the O
stars in all these binaries; thus we conclude that an efficient mechanism for
shedding angular momentum from O stars in WR + O binaries must exist. This
mechanism, probably related to Roche lobe overflow-created dynamo magnetic
fields, prevents nearly 100% breakup spin rates, as expected when RLOF
operates, as it must, in these stars. A Spruit-Tayler dynamo and O star wind
might be that mechanism.
