Illustration of the method employed for classification of detections and upper limits. The solid lines are curves of constant [Hf/Fe]_min calculated for CES 0031-1647 using Eq. (6) and a CoG model generated with MOOG for the Hf II 3399.8 Å line to convert RW_min to Hf abundances. Their colour scale corresponds to the [Hf/Fe]_min labelled in each solid line. The dotted grey line corresponds to the [Hf/Fe] measured in this work, assuming a solar Hf abundance of 0.85 dex (Asplund et al. 2009). The black square marks the point of the estimated S/N and the R_eff of CES 0031-1647, indicating that the minimum detectable abundance [Hf/Fe]_min for that star with our CERES spectrum is 0.10 dex. The unlabelled dashed line crossing that point represents [Hf/Fe]_min = 0.10 for different combinations of S/N and R_eff. Abundances to the left of the dashed line will be detections, while abundances to the right side of the dashed line will result in upper limits.