We present results on carbon , nitrogen , and oxygen abundances for a sample of unevolved metal-poor stars with metallicities in the range - 0.3 < [ Fe/H ] < -3 .
Oxygen abundances derived from different indicators are compared showing consistently that in the range 0.3 > [ Fe/H ] > -3.0 , the [ O/Fe ] ratio increases from approximately 0 to 1 .
We find a good agreement between abundances based on the forbidden line , the OH and IR triplet lines when gravities based on Hipparcos parallaxes are considered for the sample stars .
Gravities derived from LTE ionization balance in metal-poor stars with [ Fe/H ] < - 1 are likely too low , and could be responsible for an underestimation of the oxygen abundances derived using the [ O i ] line .
[ C/Fe ] and [ N/Fe ] ratios appear to be constant , independently of metallicity , in the same range .
However , they show larger scatter than oxygen at a given metallicity , which could reflect the larger variety of stellar production sites for these other elements .