Spectral syntheses for the determination of the abundances of carbon (upper panel), nitrogen (middle panel), and oxygen (lower right panel). The top panel of each plot shows the best-fit syntheses (blue lines) and uncertainties (shaded regions) compared to the observed spectra (points). Also shown are syntheses after removing all the contributions from specific elements (gray lines). The bottom panels show the residuals between the observed spectra and the syntheses. The lower left panel shows the determination of the ¹²C/¹³C ratio (see text for details).