Distribution of gravitational redshifts as a function of log T_eff. The theoretical curves are plotted as solid lines, which are labeled based on the corresponding mass and core compositions (He-, CO-, and ONe-cores Althaus et al. 2013; Camisassa et al. 2016, 2019). For reference, we also plot the models for two ultra-massive CO-core white dwarfs of 1.23 and 1.29 M_⊙ (Camisassa et al. 2022) and the relativistic models for the 1.3 M_⊙ ONe- and the 1.29 M_⊙ CO-core white dwarfs (dashed curves; Althaus et al. 2022, 2023). The observed stars are color coded accordingly to their measured masses; the adopted symbols are the same as in Fig. 8. The four labeled stars are discussed in the text.