Context : The remarkable properties of CSÂ 30322-023Â became apparent during the course of a high-resolution spectroscopic study of a sample of 23 carbon-enhanced , metal-poor ( CEMP ) stars .

Aims : This sample is studied in order to gain a better understanding of s- and r-process nucleosynthesis at low metallicity , and to investigate the role of duplicity .

Methods : High-resolution UVES spectra have been obtained , and abundances are derived using 1-D , plane-parallel OSMARCS models under the LTE hypothesis .
The derived atmospheric parameters and observed abundances are compared to evolutionary tracks and nucleosynthesis predictions to infer the evolutionary status of CSÂ 30322-023 .

Results : CSÂ 30322-023Â is remarkable in having the lowest surface gravity ( \log g \leq - 0.3 ) among the metal-poor stars studied to date .
As a result of its rather low temperature ( 4100Â K ) , abundances could be derived for 35 chemical elements ; the abundance pattern of CSÂ 30322-023Â is one of the most well-specified of all known extremely metal-poor stars .
With [ Fe/H ] = -3.5 , CSÂ 30322-023Â is the most metal-poor star to exhibit a clear s-process signature , and the most metal-poor “ lead star ” known .
The available evidence indicates that CSÂ 30322-023Â is presently a thermally-pulsing asymptotic giant branch ( TP-AGB ) star , with no strong indication of binarity thus far ( although a signal of period 192Â d is clearly present in the radial-velocity data , this is likely due to pulsation of the stellar envelope ) .
Low-mass TP-AGB stars are not expected to be exceedingly rare in a magnitude-limited sample such as the HK survey , because their high luminosities make it possible to sample them over a very large volume .
The strong N overabundance and the low ^ { 12 } C/ ^ { 13 } C ratio ( 4 ) in this star is typical of the operation of the CN cycle .
Coupled with a Na overabundance and the absence of a strong C overabundance , this pattern seems to imply that hot-bottom burning operated in this star , which should then have a mass of at least 2Â M _ { \odot } .
However , the luminosity associated with this mass would put the star at a distance of about 50Â kpc , in the outskirts of the galactic halo , where no recent star formation is expected to have taken place .
We explore alternative scenarios in which the observed abundance pattern results from some mixing mechanism yet to be identified occurring in a single low-metallicity 0.8Â M _ { \odot } Â AGB star , or from pollution by matter from an intermediate-mass AGB companion which has undergone hot-bottom burning .
We stress , however , that our abundances may be subject to uncertainties due to NLTE or 3D granulation effects which were not taken into consideration .

Conclusions :