a) The RR Lyr region of the CMD, where only variable stars detected in two filters, with well defined colours and magnitudes, are shown. Red triangles are the same as in Fig. 2a, but the colouring of the F pulsators is determined by their metallicities, where empty circles are metal-poor ( [Fe/H]  ≤  − 1.5 dex), grey circles are metal-intermediate ( − 0.5 ≥  [Fe/H]  ≥  − 1.5 dex) and filled circles are metal-rich ( [Fe/H]  ≥  − 1 dex). AC and P2C are indicated with squares and stars, respectively. AC pulsating in first-overtone mode are highlighted with filled colour (green). The coloured lines are HB tracks from (Pietrinferni et al. 2004) for different metallicities, as labeled. b) The luminosity function for our 2-filter RR Lyr sample (black solid line) compared with that found by Sarajedini et al. (2009) (red dashed line).