log P−V₀ plot of the SX Phe stars in M 68, with the P−L relation of Cohen & Sarajedini (2012) for fundamental, first-overtone, and second-overtone pulsators as solid, dotted, and dashed lines, respectively. Shaded areas show 1-σ boundaries for each case. The relations have been shifted to a distance of 9.99 ± 0.21 kpc (see Sect. 6.4.1), and the SX Phe magnitudes have been dereddened using our adopted value of E(B−V) = 0.05 ± 0.01 mag. The location of the black filled circles in this plot correspond to the periods listed in Table 3, and for V50 the first-overtone period is shown as a triangle with a dash-dotted line connecting the two periods.