Upper panel: Linear least-squares fit to 11 periods of WD 1310+583 marked with asterisks in Appendix A. The derived period spacing from this fit is ΔΠ_ℓ = 1 = 40.51 s. Lower panel: Residuals of the period distribution relative to the mean period spacing, revealing signals of mode trapping in the period spectrum of WD 1310+583. Modes with a consecutive radial order are connected with thin black lines.