  We present data for four ultra-Li-deficient, warm, halo stars. The Li
deficiency of two of these is a new discovery. Three of the four stars have
effective temperatures Teff ~ 6300 K, in contrast to previously known
Li-deficient halo stars which spanned the temperature range of the Spite
Plateau. In this paper we propose that these, and previously known
ultra-Li-deficient halo stars, may have had their surface lithium abundances
reduced by the same mechanism as produces halo field blue stragglers. Even
though these stars have yet to reveal themselves as blue stragglers, they might
be regarded as "blue-stragglers-to-be." In our proposed scenario, the surface
abundance of Li in these stars could be destroyed (a) during the normal
pre-main-sequence single star evolution of their low mass precursors, (b)
during the post-main-sequence evolution of a evolved mass donor, and/or (c) via
mixing during a mass-transfer event or stellar merger. The warmest Li-deficient
stars at the turnoff would be regarded as emerging "canonical" blue stragglers,
whereas cooler ones represent sub-turnoff-mass "blue-stragglers-to-be." The
latter are presently hidden on the main sequence, Li depletion being possibly
the clearest signature of their past history and future significance.
Eventually, the main sequence turnoff will reach down to their mass, exposing
those Li-depleted stars as canonical blue stragglers when normal stars of that
mass evolve away. Arguing against this unified view is the observation that the
three Li-depleted stars at Teff ~ 6300 K are all binaries, whereas very few of
the cooler systems show evidence for binarity; it is thus possible that two
separate mechanisms are responsible for the production of Li-deficient
main-sequence halo stars.
