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The isotope abundances provide powerful diagnostics of the chemicalenrichment in our Galaxy. The star HD 140283 is one of the best-studied verymetal-poor dwarf stars. It is very old, and the chemical abundance in this staris a good witness of the chemical composition of the matter in the earlyGalaxy. The aim of this work is to measure the precise abundances of carbon,nitrogen, oxygen, and mainly the 12C/13C isotopic ratio in this very oldmetal-poor star in order to have a good reference for the computations of thechemical evolution of the Galaxy. We used very high spectral resolution data,with extremely high signal-to-noise ratios obtained with the spectrographsESPaDOnS at the CFHT, ESPRESSO at the VLT, and HARPS at the ESO 3.6m telescope.For the first time, we were able to measure the 12C/13C ratio in a very oldmetal-poor dwarf that was born at the very beginning of the Galaxy: 27 <12C/13C < 45. We also obtained a precise determination of the abundance of theCNO elements in this star. These abundances suggest that the effect ofsuper-asymptotic giant branch stars or fast-rotating massive stars wassignificant in the early Galaxy.

12C/13C ratio and CNO abundances in the classical very old metal-poor dwarf HD 140283