CNO at the surface of low and intermediate MS stars
  Recent studies of spectral lines of the CNO elements in the Sun and unevolved disk and halo stars show that most published abundance data may be affected by systematic errors due to inadequate 1D modelling of stellar atmospheres. This raises doubts about previously derived trends of CNO abundances as a function of [Fe/H]. In this review we concentrate on abundance ratios derived from weak spectral features with similar dependences on temperature and pressure, which ratios are then relatively insensitive to 3D model atmosphere effects and to effective temperature and surface gravity. The recent controversy about the trend of [O/Fe] vs. [Fe/H] is discussed, new results for C/O ratios as determined from forbidden lines and high excitation neutral lines are presented, and N/C ratios derived from NH and CH lines are commented on. 
Poul E. Nissen