The ratio v_∞/v_esc as a function of the stellar parameters L_∗, R_∗, T_eff, and v_esc. Compared to the theoretical predictions of Pauldrach et al. (2004, P04; CSPNs), this work, and (Pauldrach et al. 2012, P11; massive O stars) the values deduced from observations and theoretical relations are shown for our sample of CSPNs (Kudritzki et al. 1997, K97) and a sample of massive O stars (Repolust et al. 2004, R04). The two filled squares shown in the plots depict the two consistent CSPN models for NGC 6826 and NGC 2392 which have been computed on the basis of the parameter sets of K97. The filled triangle represents an additional consistent model for NGC 6826 where the surface gravity has been reduced to a value of log g = 3.7 (the parameters of the models are listed in Table 2). For a discussion see the text.