HOT DEGENERATES IN THE MONTREAL-CAMBRIDGE-TOLOLO SURVEY .2. 2 NEW HYBRID WHITE-DWARFS, MCT-0128-3846 AND MCT-0453-2933, AND THE NATURE OF THE DAB STARS

We report the discovery, in the course of a survey for high-latitude, ultraviolet-excess objects in the southern hemisphere, of two new hybr id DAB stars. Both objects exhibit a weak He I lambda4471 line superpo sed onto the usual Balmer line spectrum. Analyses of the Balmer series with both homogeneous and stratified models show these stars to have spectroscopic effective temperatures around 27,000 K. However, low-dis persion ultraviolet observations with the IUE reveal energy distributi ons considerably flatter than those of normal DA stars near that effec tive temperature. These energy distributions are inconsistent as well as those of stratified models calculated at the temperature and hydrog en-layer mass determined from the optical spectrum. We demonstrate ins tead that consistent fits to the optical and ultraviolet spectrum, as well as to the energy distribution, can be achieved if these objects a re unresolved, composite systems consisting of a DA white dwarf togeth er with a DB or DBA star. The DAB spectral class thus appears inhomoge neous, and may contain both composite systems and stratified stars.