Element abundance ratios in an extremely metal-poor binary star: CS 22873-139

High-resolution spectra of the extremely metal-poor double-lined spectrosco pic binary star BPS CS 22873-139 have been analyzed to determine the metall icity and abundance ratios for a number of elements. From the analysis of t he collected radial velocities, new orbital elements are derived as well as an improved mass ratio. In spite of its extreme metal deficiency, [Fe/H]= -3.4, the abundance ratios of CS 22873-139 are not similar to those of a ty pical Population II star. In particular, the cu-elements are not enhanced r elative to iron, and the strontium abundance is very low. The abundance pattern of CS 22873-139 is compared to the patterns exhibited by other metal-poor stars which exhibit also very low strontium abundances . There is a large spread of elemental abundance ratios among these stars, suggesting that low strontium abundance may be associated with a variety of nucleosynthesis histories. The abundance ratios of CS 22873-139 are, surpr isingly, very similar to those found in a common proper-motion pair HD 1344 39, HD 134440, even though the metallicity of this system is almost two dex higher, [Fe/H] = -1.7. The ratios are compared to those of another very me tal-poor binary star BPS CS 22876-032. The unusual abundance pattern of CS 22873-139 is discussed (by comparison to the predicted yields of zero-metal SN II and hypernovae). The lithium doublet at 6707 Angstrom is not detected in CS 22873-139, but a n abundance of lithium consistent with the Spite plateau cannot be excluded , based on the present data.