Sample objects illustrated in the Kiel diagram. The dotted lines are evolutionary tracks of H-rich WDs (Z = 0.01) from Renedo et al. (2010), representing in order 0.525, 0.570, 0.593, 0.609, 0.632, 0.659, 0.705, 0.767, and 0.837 M_⊙. The solid and dashed black lines correspond to the theoretical wind limit and to the He abundance (N(He)/N(H) = 10⁻³) calculated with the predicted mass-loss rates by Unglaub & Bues (2000, see their Fig. 6), respectively. The latter also coincides with the approximate optical detection limit of He.