  Observations of eighteen halo main-sequence-turnoff stars, four of which are
extremely deficient in Li, show that three of the Li-poor ones have substantial
line broadening. We attribute this to stellar rotation. Despite the great ages
of halo stars, for G202-65, BD+51:1817 and Wolf 550 we infer vsini = 8.3 +/-
0.4, 7.6 +/- 0.3, and 5.5 +/- 0.6 km/s respectively. The stated errors are
3sigma. For CD-31:19466 we derive a 3sigma upper limit vsini < 2.2 km/s. The
three rotating stars are known spectroscopic binaries. We explain the high
rotation velocities in terms of mass and angular momentum transfer onto the
surface of the turnoff star from an initially more-massive donor. Estimates of
the specific angular momentum of accreted material indicate that quite small
transfer masses could have been involved, though the unknown subsequent
spin-down of the accretor prevents us from assigning definitive values for each
star. The accretor is now seen as an ultra-Li-deficient star whose origin makes
it a low-mass counterpart of field blue stragglers. The Li could have been
destroyed before or during the mass-transfer episode. Such objects must be
avoided in studies of the primordial Li abundance and in investigations into
the way normal single stars process their initial Li.
