We report high-resolution , high-signal-to-noise , observations of the extremely metal-poor double-lined spectroscopic binary CS 22876–032 .
The system has a long period : P = 424.7 \pm 0.6 days .
It comprises two main sequence stars having effective temperatures 6300 K and 5600 K , with a ratio of secondary to primary mass of 0.89 \pm 0.04 .
The metallicity of the system is [ Fe/H ] = –3.71 \pm 0.11 \pm 0.12 ( random and systematic errors ) – somewhat higher than previous estimates .

We find [ Mg/Fe ] = 0.50 , typical of values of less extreme halo material .
[ Si/Fe ] , [ Ca/Fe ] , and [ Ti/Fe ] , however , all have significantly lower values , \sim 0.0–0.1 , suggesting that the heavier elements might have been underproduced relative to Mg in the material from which this object formed .
In the context of the hypothesis that the abundance patterns of extremely metal-poor stars are driven by individual enrichment events and the models of Woosley and Weaver ( 1995 ) , the data for CS 22876–032 are consistent with its having been enriched by a zero-metallicity supernova of mass 30 M _ { \odot } .

As the most metal-poor near-main-sequence-turnoff star currently known , the primary of the system has the potential to strongly constrain the primordial lithium abundance .
We find A ( Li ) ( = log ( N ( Li ) /N ( H ) ) + 12.00 ) = 2.03 \pm 0.07 , which is consistent with the finding of Ryan et al .
( 1999 ) that for stars of extremely low metallicity A ( Li ) is a function of [ Fe/H ] .