PLD of O-rich (top panels) and C-rich (bottom panels) LPVs pulsating only in the FM. Panels in the left and central columns show, respectively, SRV-0⁺ and SRV-0⁻ stars (cf. Fig. 5). The former tend to align on sequence C, as Miras do (panels in the right columns), while the latter often lie on sequence F, between sequences C′ and C. Solid and dashed lines are best-fit PLRs from Soszyński et al. (2007) (cf. Fig. 1). As they do not provide a fit to sequence F, to indicate its approximate location, we used the best-fit equation for sequence C and offset it by 0.65 mag towards brighter W_J, Ks (dash-dot line).