1997A&A...324L..53H

HS 0209+0832, the only white dwarf known to show helium lines in the DB gap region, is reported to display profile variations of helium lines. A quantitative spectroscopic analysis using NLTE model atmospheres reveals that the line profile variations are consistent with a constant T_eff=36100K and log g=7.91 but with a varying helium abundance. It decreased from a high abundance state (He/H=0.012, by number) by a factor of 2 to 3 in 1995 and recovered back to the high abundance state in 1996. The previously suggested possibility that HS0209+0832 is a composite spectrum (DA+DB) object is ruled out by the spectral analysis. Hence HS0209+0832 is the first DAB star to be unambigously near the middle of the DB gap and appears to be explainable by a homogeneous He/H atmosphere with the helium abundance lower by a factor of 2-3 in one observation.