Dipole (ℓ = 1) asymptotic period spacing, , corresponding to each PG 1159 evolutionary sequence (dashed and solid curves) in terms of the logarithm of the effective temperature. Numbers along each curve denote the stellar mass (in solar units). Dashed (solid) lines correspond to evolutionary stages before (after) the turning point at the maximum effective temperature of each track (the evolutionary knee). The observed period spacing, , derived for SDSS J0349−0059 and VV 47, is depicted with horizontal solid lines. The uncertainties in and T_eff for each star are also indicated.