HR diagram in G absolute magnitude (M_G0) and dereddend (G_BP − G)₀ colour. Left panel: M_G0 (G_BP−G)₀ CMD of 915 DR3 RRab stars that have σ_ϖ/ϖ< 0.1, RUWE < 1.4, and A(G) < 0.2; right panel: same as in the left panel, but for 620 RR Lyrae of the sample for which A(G) < 0.1. The sources are colour-coded according to their metallicity. The lines show the boundaries of theoretical ISs for RRab stars of three different chemical compositions (Z = 0.0001, solid lines; Z = 0.001, dashed lines; and Z = 0.02, dotted lines) from Marconi et al. (2015).