K_s vs. J − K_s colour–magnitude diagram of stars on the upper giant branch of ω Cen. Known LPVs from Tables 1 and 2 are marked by large circles while smaller points are stars in a box of side 500 arcsec centred on the cluster and corresponding approximately to the field we monitored for LPVs. For the LPVs, values of [Fe/H] from Tables 1 and 2 were used to colour the circles according to the key shown on the figure. For stars not known to be variable, when an estimate of [Fe/H] existed from Suntzeff & Kraft (1996), this metallicity was used to colour the smaller circle.