Upper panel: linear least-squares fit to the 13 periods of GD 358 marked with asterisks in Table 3. The derived period spacing from this fit is ΔΠ = 39.25 ± 0.17 s. Lower panel: residuals of the period distribution relative to the mean period spacing, revealing signals of mode trapping in the period spectrum of GD 358. Modes with a consecutive radial order are connected with thin black lines.