Top panel: chromosome map of M 2. The P1 and P2 stars in the main cluster component are displayed as large orange circles and black crosses, respectively, while the P1 metal-rich component is shown as green squares. Middle panel:F814W-(F275W − F814W) CMD of the three groups of stars shown in the chromosome map. Bottom panel: similar to the right panel of Fig. 2, but the orange circles display the difference of the (F275W − F814W) colour between the P1 stars of the main cluster component, and the ‘verticalised’ fit to the CMD of the metal-rich component is considered as a reference (dashed line). The individual metal-rich P1 stars are also plotted around the verticalised fit.