Observed spectrum of RY Cru (solid black line, left and centre panel) and Gaia DR2 4644478166748030976 (solid orange line, right panel) in the region of DyII (5090.386 Å, left panel), ErII (5028.905 Å, centre panel) and LuII (6221.89 Å, right panel) spectral lines. The position of these and other visible lines is reported and highlighted with vertical dashed grey lines. Synthetic spectra computed for the best estimated abundance are reported in blue, while the same spectra with [X/H]=±0.5 dex are plotted in red and green, respectively.