The DA white dwarf Sirius B has gained special interest because of the unexplained observed X-rays (Mewe et al. 1975). Coronal emission, accretion and normal photospheric radiation have been discussed as sources.
A DA white dwarf with Teff ∽ 28000K (Savedoff et al. 1976, 1978, Koester 1979) should not have a corona (Böhm and Cassinelli 1971, Fontaine 1973). For accretion we do not see where the accreted mass should come from. Normal photospheric radiation requires an effective temperature around 32000K or higher (Shipman 1976). All possibilities to explain the X-rays therefore encounter severe problems.