  The isotope abundances provide powerful diagnostics of the chemical
enrichment in our Galaxy. The star HD 140283 is one of the best-studied very
metal-poor dwarf stars. It is very old, and the chemical abundance in this star
is a good witness of the chemical composition of the matter in the early
Galaxy. 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 old
metal-poor star in order to have a good reference for the computations of the
chemical evolution of the Galaxy. We used very high spectral resolution data,
with extremely high signal-to-noise ratios obtained with the spectrographs
ESPaDOnS 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 old
metal-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 the
CNO elements in this star. These abundances suggest that the effect of
super-asymptotic giant branch stars or fast-rotating massive stars was
significant in the early Galaxy.
