  We present a detailed abundance analysis, including spectral syntheses, of a
very metal-poor ([Fe/H]= -2.7), peculiar main sequence star, HE0024-2523
detected during the course of the Keck Pilot Program. Radial velocities of this
star were obtained during four different observing runs over a time span of 1.1
years, and demonstrate that it is clearly a short period spectroscopic binary.
An orbital solution was obtained, and orbital parameters were determined with
high precision. The rotational velocity was also measured (vsin i=9.7$\pm$1.5
kms); rotation appears likely to be synchronous with the orbit. The abundance
analysis and spectral syntheses indicate that the object is a CH star
characterized by extreme s-process enrichment, likely due to mass accretion
from an evolved companion which has now probably become a white dwarf. The lead
(Pb) abundance of HE0024-2523 is very high, the same as that of the recently
discovered lead-rich metal-poor star CS 29526-110, [Pb/Fe]=+3.3. The abundance
ratio of the heavy-s to light-s elements, as characterized by Pb and Ba,
[Pb/Ba]=+1.9, is the highest yet found for any metal-poor star, and is about
0.7 dex higher than that of CS29526-110. On the basis of the measured isotopic
ratio of carbon (12C/13C about 6) we argue that the mass donor must have had an
original mass of at least 3 Msun. The unusually short period of this CH star
suggests that it underwent a past common-envelope phase with its evolved
companion. Our results are compared to the latest available models for AGB
yields and s-process nucleosynthesis. We also discuss the possible connection
between HE0024-2523 the lithium depletion of halo stars, and halo blue
straggler formation.
