Upper panel: Na-O anticorrelation among NGC 1851 RGB stars from HR spectroscopy studies. Red color refers to stars photometrically selected to belong to the red-RGB in the Strömgren u, (u − y) + (v − b) diagram, while stars located on the blue-RGB are shown in blue. Symbols and color code are consistent with those of Fig. 12. Bottom panel: the run of [La/Fe] versus [Fe/H] and the normalized number distribution for red and blue stars in this plane. The color code is consistent with the upper panel.