The hot DO white dwarf HD 149499 B - Einstein redshift of a DB progenitor with carbon features

High-resolution IUE spectra of the hot DO white dwarf HD 149499B have been analyzed. The mass of the white dwarf needed to fit the measured gravitational redshift is 0.4-0.6 solar mass for a degenerate core composed of carbon based upon using a nonzero temperature mass-radius relation. The resonance doublet of C IV (1550 A) appears to be photospheric in origin, and two possible explanations for the presence of carbon in a hot DO white dwarf are discussed. The relationship of HD 149499B and other DO white dwarfs to the DB white dwarfs is also considered.