Map of the total sample of RRab stars used in this work. The red and green circles are the RRab stars discovered using the VVV1 and VVV2 catalogues, respectively; the blue circles are from the OGLE catalogue; and the grey circles are from Gaia DR2. The irregular pattern for the Gaia catalogue is a consequence of its spinning and precessing pattern of observation in the sky (known as the Gaia scanning law; see e.g. Boubert et al. 2020).