Detections of calcium in two DBA stars and in two DB stars as well as possible detections of calcium and/or hydrogen in three additional DB degenerates, from moderate-resolution spectra obtained at the Multiple Mirror Telescope are reported. Calcium and hydrogen abundances and abundance limits which are close to the cosmic abundance have been derived. This finding would appear to indicate that accretion of interstellar hydrogen and grains can effectively occur in some single helium-rich degenerates up to effective temperatures of at least 20,000 K. If a 'propeller' mechanism is operative in DB stars, the results would suggest that the mechanism may break down or be less efficient in a significant fraction of these objects.
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Sion et al. (1988) studied this question.