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tokens: detected in the spectra, as illustrated for two objects in Figure 9 . In such instances, we simply excluded this narrow wavelength region from the fits. We are confident that ignoring the carbon opacity in our models only marginally impacts the derived values of and log g . Indeed, the carbon abundances measured in DOZ white dwarfs typically fall in the range −3.0 log C/He −2.0 (Dreizler Werner 1996 ; Reindl et al. 2014a ; Werner et al. 2014 ), and the corresponding opacity barely influences the He i and He ii line profiles at high temperatures according to the model-atmosphere calculations of Reindl et al. ( 2018 ). Figure 8. Best model-atmosphere fits to the optical spectra of DA white dwarfs with high signal-to-noise observations (S/N > 30). The observed and synthetic spectra, normalized to a continuum set to unity, are displayed as black and red lines, respectively. The fits are offset vertically from each other by 0.6 for clarity. The SDSS name, the spectral type, and the best-fitting atmospheric parameters are given for each object. The positions of the H i lines are marked by blue ticks. Figure 9. Same as Figure 8 , but for DO/DB white dwarfs. The positions of the He i and He ii lines are marked by green and magenta ticks, respectively. 4.2. Hybrid White Dwarfs Special care was needed to properly analyze the 127 stars in our sample exhibiting both hydrogen and helium spectral features. As indicated by Figure 5 , the great sensitivity of the spectrum to the chemical structure of the atmosphere can be exploited to discriminate between homogeneous and stratified distributions of hydrogen and helium (Bergeron et al. 1994 ; Manseau et al. 2016 ). To this end, we fitted the optical spectra of our hybrid white dwarfs with both our homogeneous and stratified model-atmosphere grids. The physical quantity used to measure the composition (the helium-to-hydrogen number ratio He/H in the homogeneous case, the fractional mass of the hydrogen layer q H in the stratified case) was treated as an additional free parameter in the optimization process. We then compared the two solutions and adopted the best-fitting one. Figure 10 contrasts the homogeneous and stratified solutions for four hybrid white dwarfs in our sample. Once again, while only the blue optical spectra are displayed here, redder wavelengths, up to λ = 7000 Å, were included in the fits as well. For each star, the parameters corresponding to the adopted chemical configuration are highlighted in red. The top two white dwarfs are relatively hot, in which situation the comparison rests largely on the He ii λ 4686 line and to a lesser extent on the He i λ 4471 line. We can safely assert that the first object has a homogeneous atmosphere, because our stratified models produce a He ii λ 4686 feature that is too shallow. Conversely, the second object bears the signature of a stratified atmosphere, the He ii λ 4686 profile predicted by our homogeneous models being too deep. The bottom two