I-V (left panel) and K-S (right panel) significance tests to search for a constant period spacing in GD 358. The tests were applied to the complete set of 352 pulsation periods of Tables 2 and 3 of Bischoff-Kim et al. (2019) (blue curves), and to the combination of the TESS periods plus the mean periods of Bischoff-Kim et al. (2019) (green curves), included in Table 3. A clear signal of a constant period spacing at ∼39 s is evident. See text for details.