The average of the computed period spacings corresponding to our DB white dwarf sequences with different stellar masses. The location of KIC 8626021 is shown with a red star symbol (T_eff = 24 900 ± 750 K,  s). By simple linear interpolation, we found that the mass of the star according to its period spacing is M_∗ = 0.696  ±  0.031 M_⊙. According to spectroscopy (log g = 7.91  ±  0.07; see Fig. 1), the period spacing of the star should be somewhere within the dark shaded area.