Asymptotic period spacing, ΔΠ^a, for dipole modes (ℓ = 1) as a function of the effective temperature for the ONe evolutionary cooling sequences with masses 1.10, 1.16, 1.22, and 1.29 M_⊙. Crystallization was computed taking into account latent heat release and chemical rehomogenization caused by phase separation. The palette of colors corresponds to the fraction of crystallized mass (M_c/M_⋆). Vertical dotted lines show the T_eff interval of the ZZ Ceti instability strip.