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We present a detailed analysis of 108 helium-line (DB) white dwarfs based onmodel atmosphere fits to high signal-to-noise optical spectroscopy. We derive amean mass of 0.67 Mo for our sample, with a dispersion of only 0.09 Mo. Whitedwarfs 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 ourprevious investigation, we find no evidence for the existence of low-mass (M <0.5 Mo) DB white dwarfs. We derive a luminosity function based on a subset ofDB white dwarfs identified in the Palomar-Green survey. We show that 20% of allwhite dwarfs in the temperature range of interest are DB stars, although thefraction drops to half this value above Teff ~ 20,000 K. We also show that thepersistence of DB stars with no hydrogen features at low temperatures isdifficult to reconcile with a scenario involving accretion from theinterstellar medium, often invoked to account for the observed hydrogenabundances in DBA stars. We present evidence for the existence of two differentevolutionary channels that produce DB white dwarfs: the standard model where DAstars are transformed into DB stars through the convective dilution of a thinhydrogen layer, and a second channel where DB stars retain a helium-atmospherethroughout their evolution. We finally demonstrate that the instability stripof pulsating V777 Her white dwarfs contains no nonvariables, if the hydrogencontent of these stars is properly accounted for.

A Comprehensive Spectroscopic Analysis of DB White Dwarfs