Figure 3 : We present here the log g – Teff diagram for our sample. White dwarfs with He+C, homogeneous He+H and stratified He+H atmospheric compositions are shown in blue, red and green, respectively. The black lines represent hydrogen@deficient evolutionary sequences from Althaus et al. (2009), labeled by stellar mass (in solar masses). We note that almost all carbon@polluted objects have Teff > 60,000 K, which indicates that gravitational settling removes carbon from the atmosphere with cooling. Also, the scatter in log g values increases at Teff > 70,000 K, probably because of the lower sensitivity of He II lines to surface gravity at high temperatures. Furthermore, white dwarfs with stratified atmospheres are all found in the range 50,000 K > Teff > 40,000 K, which corresponds to the hot end of the so@called DB gap (a range in effective temperature, 45,000 K > Teff > 30,000 K, where a significant deficit of helium@rich white dwarfs is observed). Thus, these objects are probably undergoing the DO to DA transition, as residual hydrogen (previously mixed in the envelope) gradually diffuses upward. Finally, one prominent feature of this diagram is the tendency for low log g values at Teff < 50,000 K, which we discuss in the next figure caption. A)Non-LTE)Model)Atmosphere)Analysis)of)Hot,)Hydrogen-Deficient)White)Dwarfs Antoine)Bédard*,)Pierre)Bergeron,)Gilles)Fontaine Université de)Montréal,)*bedard@astro.umontreal.ca White dwarf stars of spectral type DO populate the hot end of the hydrogen-deficient white dwarf cooling sequence. They are characterized by a helium-rich atmosphere, a high effective temperature (Teff > 40,000 K) and a high surface gravity (log g > 7), and thus their spectrum is dominated by broad He II absorption features. Their hydrogen deficiency is thought to result from a so-called born-again evolution, during which a post-AGB star undergoes a late helium-shell flash, bringing the star back to the AGB and causing the remaining hydrogen to be burned or completely mixed in the envelope. In order to further our understanding of hydrogen-deficient white dwarf evolution, we perform a model atmosphere analysis of a large sample of DO stars. Atmospheric parameters are derived by comparing optical spectra to our new grid of non-LTE model atmospheres and synthetic spectra. We also make use of evolutionary sequences to obtain stellar masses and cooling ages. Based on these results, we discuss the physical properties of DO white dwarfs. Figure 1 : This figure shows the best fits to the optical spectra of some DO white dwarfs in our sample (each offset by 0.5), together with the corresponding Teff and log g values. The observed and theoretical spectra are displayed as black and red lines, respectively. Our sample is composed of 103 DO stars observed spectroscopically in the Sloan Digital Sky Survey (SDSS), and 14 bright DO stars whose spectra were secured at the Steward Observatory 2.3 m Bok telescope equipped with the Boller & Chivens spectrograph. To analyze these observations, we computed a new grid of non@ LTE, helium@rich model atmospheres and synthetic spectra using the codes TLUSTY and SYNSPEC (Hubeny & Lanz 1995). The best@fitting atmospheric parameters were obtained by first normalizing the observed and model spectra to a continuum set to unity, and then applying a χ2 minimization procedure to these normalized spectra. Figure 2 : This figure is similar to Figure 1, but for some white dwarfs exhibiting distinctive spectral features. 31 objects, such as SDSS J011346.77+002828.6, show traces of carbon. For these stars, the C IV !4658, !5801 and !5812 lines (indicated by thick marks) are simply excluded from the fits. Also, 14 objects, such as SDSS J101355.61–001396.4 and SDSS J003343.05+142251.4, contain hydrogen, either in a homogeneous or stratified configuration. In the former case, we infer the hydrogen abundance by number (NH/NHe), while in the latter case, we constrain the mass of the hydrogen layer floati