Period-amplitude diagrams of O-rich stars, colour coded by evolutionary group to which they belong. The green dots indicate faint AGBs and RGBs, the blue dots indicate low-mass AGBs, and the cyan squares indicate intermediate-mass AGBs. The massive AGBs and RSGs from branch (d) are shown as orange/red diamond symbols according to whether they belong to sequence C′/C in the P–W_BP, RP diagram. The three panels present the amplitudes in G, G_BP, and G_RP, respectively, taken from the magnitude ranges published in Gaia DR2.