Comparison between observed (upper panel) and a synthetic HB for NGC 1851. The synthetic CMD has been obtained by considering:  ⟨ M ⟩  = 0.670 ± 0.005 M_⊙, Y = 0.248 (red open squares);  ⟨ M ⟩  = 0.640 ± 0.006 M_⊙, Y = 0.265 (filled red circles);  ⟨ M ⟩  = 0.650 ± 0.004, M_⊙, Y = 0.248 (cyan filled triangles);  ⟨ M ⟩  = 0.590 ± 0.005 M_⊙, Y = 0.280 (blue open circles). These populations have been selected in order to reproduce the observed distribution of stars on the HB and SGB, as well as the anomalous red sequence on the RGB in the (v,(v − y)) diagram (see text for further details). Solid and dashed lines in the upper panel represent the average brightness of, respectively, the horizontal part of the blue HB and the red HB, as derived from the synthetic CMD. For ease of comparison, the dashed line is also displayed at the colour range of the horizontal part of the blue HB.