  New boron abundances or upper limits have been determined for 8 early-B stars
in the young Galactic open cluster NGC 3293, using ultraviolet spectra obtained
by the Hubble Space Telescope Cosmic Origins Spectrograph. With previous
observations, there are now 18 early-B stars in this cluster with boron
measurements. Six of the newly observed stars have projected rotational
velocities greater than 200 km/s, allowing new constraints on rotationally
driven mixing in main-sequence stars. When comparing to synthetic model
populations, we find that the majority of our sample stars agree well with the
predicted trends of stronger boron depletion for larger rotation and for larger
mass or luminosity. Based on those, a smaller than the canonical rotational
mixing efficiency,(fc = 0.0165 vs the more standard value of 0.033), appears to
be required. However, our five most slowly rotating stars are not well
explained by rotational mixing, and we speculate that they originate from
binary mergers.
