Quality function of the period fit in terms of the effective temperature for the PG 1159 sequences with different stellar masses (in solar units), indicated at the left bottom corner of each panel. Black (red) lines correspond to stages before (after) the evolutionary knee (see Fig. 1). Only the periods with m = 0 (see Table 9) have been considered. The strong minimum in panel c corresponds to M_⋆ = 0.565 M_⊙. Panel h is a zoom of the region with the strong minimum seen in panel c; the y-axis scale is linear in this case. The vertical dashed line is the spectroscopic T_eff of RX J2117 (170 kK), and the gray zone depicts its uncertainties (±10 kK).