UBV PHOTOMETRY OF HOT WHITE-DWARF STARS

Johnson UBV photometry has been obtained for a set of hot degenerate s tars, primarily DA and DO white dwarfs from among those detected in th e 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 a lso x-ray and extreme ultraviolet sources from the ROSA T all sky surv ey. 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 difficul ties in defining the atmospheric cutoff of the U band and a general la ck of hot stars used to define the photometric transformation between theoretical and observed colors.