We present a detailed analysis of 108 helium-line (DB) white dwarfs based on model atmosphere fits to high signal-to-noise optical spectroscopy. We derive a mean mass of 0.67 M ⊙ for our sample, with a dispersion of only 0.09 M ⊙ . White dwarfs also showing hydrogen lines, the DBA stars, comprise 44% of our sample, and their mass distribution appears similar to that of DB stars. As in our previous investigation, we find no evidence for the existence of low-mass (M < 0.5 M ⊙ ) DB white dwarfs. We derive a luminosity function based on a subset of DB white dwarfs identified in the Palomar-Green survey. We show that 20% of all white dwarfs in the temperature range of interest are DB stars, although the fraction drops to half this value above T eff ∼ 20, 000 K. We also show that the persistence of DB stars with no hydrogen features at low temperatures is difficult to reconcile with a scenario involving accretion from the interstellar medium, often invoked to account for the observed hydrogen abundances in DBA stars. We present evidence for the existence of two different evolutionary channels that produce DB white dwarfs: the standard model where DA stars are transformed into DB stars through the convective dilution of a thin hydrogen layer, and a second channel where DB stars retain a helium-atmosphere throughout their evolution. We finally demonstrate that the instability strip of pulsating V777 Her white dwarfs contains no nonvariables, if the hydrogen content of these stars is properly accounted for.

Bolometric corrections and color indices on various photometric systems are provided for an extensive grid of hydrogen-and helium-rich white-dwarf model atmospheres. Absolute visual magnitudes, masses, and ages are also obtained for each model from detailed evolutionary cooling sequences appropriate for these stars. The results of our calculations are briefly compared with published observational material. These calculations can easily be extended to any given photometric system.

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We have secured optical spectra for the eight currently known variable DB, or V777 Her, stars. With the help of a new generation of synthetic spectra, spectroscopic e †ective temperatures are derived for these objects, as well as for 15 other DB or DBA stars above 20,000 K. We Ðnd that the location of the boundaries of the instability strip is sensitive to the atmospheric hydrogen abundance assumed for DB stars : the strip covers the range 22,400È27,800 K if atmospheres of pure helium are used and the range 21,800È24,700 K if undetectable traces of hydrogen are allowed for in the DB models. These determinations provide independent constraints for current seismological analyses of the V777 Her stars. More sensitive searches for weak hydrogen features in hot DB stars should help decide between the two temperature scales.

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