UBV PHOTOMETRY OF HOT WHITE-DWARF STARS

Johnson UBV photometry has been obtained for a set of hot degenerate stars, primarily DA and DO white dwarfs from among those detected in the Palomar-Green survey of UV excess objects. Most of our program stars have estimated effective temperatures (T(eff)) in the range 22 000 to 80 000 K and have no previous photometry. Some objects selected are also x-ray and extreme ultraviolet sources from the ROSA T all sky survey. The importance of precise photometric measurements in the analysis of x-ray data is discussed. A discrepancy between the observed colors and predicted colors is noted, and possibly accounted for by difficulties in defining the atmospheric cutoff of the U band and a general lack of hot stars used to define the photometric transformation between theoretical and observed colors.