V − I,V colour–magnitude diagram of NGC 1904 extracted from the reference images. The locations of the confirmed RR Lyrae variables with V and I photometry are marked with blue filled circles (RR0 variables) and blue filled triangles (RR1 variables), V7 with a green filled circle, and V1, for which we do not find evidence of variability, with a red cross. Note that V5 and V10 lie above the horizontal branch because they are significantly blended, as discussed in the text. Error bars for various magnitude levels are shown near the right edge of the plot. Also shown are an interpolation of the VandenBerg et al. (2006) isochrones for a metallicity of  [Fe/H] _ZW = −1.63 (see Sect. 5.2) at ages of 9, 10, 11, 12, 13, 14 and 15 Gyr (marked by lines of different colours; see Sect. 5.3).