Comparison of CNO elemental abundances of mixed and unmixed stars with HD 20 shown in blue for comparison. Gray circles resemble the study by Gratton et al. (2000) while red triangles indicate the stars published in Spite et al. (2005, 2006). Two C-rich stars were excluded from the latter sample. Lower limits on ¹²C/¹³C are indicated by upward pointing arrows and the classification into mixed and unmixed stars according to the authors are represented by open and filled symbols, respectively. The red line in the lower panel mimics the linear relation between [N/O] and [O/H] for mixed stars as reported by Spite et al. (2005), whereas the dashed line extrapolates the same relation to higher values of [O/H].