We present a high-resolution spectroscopic analysis of the extremely-metal-poor star CS 22957-027 , which Beers et al .
( 1992 ) noted to exhibit a rather strong G-band .
For { T } _ { eff } ~ { } = 4839 K , derived from broadband photometry , our analysis obtains \log g = 2.25 , and a very low metallicity of [ Fe/H ] = -3.43 \pm 0.12 .
The carbon-to-iron ratio is found to be enhanced by \approx 2 dex relative to the solar value , similar to the handful of other carbon–rich metal–deficient stars discussed by Norris et al .
( 1997a ) and Barbuy et al .
( 1997 ) .
From the ^ { 12 } C ^ { 13 } C ( 1,0 ) Swan bandhead and ^ { 13 } CH lines the isotope ratio ^ { 12 } C / ^ { 13 } C is \approx 10 , indicating a ^ { 13 } C enrichment .
Nitrogen is also found to be enhanced by \approx 1 dex .
The s - process elements Sr and Ba are , surprisingly , found to be under-abundant relative to solar , with [ Sr/Fe ] =-0.91 and [ Ba/Fe ] =-0.93 .
The star ’ s low luminosity requires that the chemical enrichment arises from mass transfer from an evolved companion , rather than self-polluting dredge-up processes .
However , the s - process elements Sr and Ba are unusually low , with a solar ratio to one another , at variance with what found in the “ classical ” CH stars .

Finally we note that a feature due to ^ { 13 } CH is present in coincidence with the Th II 401.9129 nm resonance line .
The blend is of relevance for the nucleo-chronology use of the Th II line in these extremely metal poor stars , when significant ^ { 13 } C is present .