It has already been shown that most, and probably all, of the DA white dwarfs become variable in a narrow temperature range as they cool down (Fontaine et al. 1982). Optical photometry and spectrophotometry has led to several determinations of the boundaries of this instability strip. The strip has been found to cover the range 10300 - 13600 K (McGraw 1979), 10400 - 12100 K (Greenstein 1982), 10000 - 13000 K (Weidemann and Koester 1984) and 11000 - 13000 K (Fontaine et al. 1985). Theoretical calculations show that the location of the blue edge is very sensitive to the efficiency of convection used in the unpertubed models (Winget et al. 1982; Winget and Fontaine 1982; Fontaine, Tassoul, and Wesemael 1984). Also, the sharpness of this boundary depends on the range of stellar mass and thickness of the hydrogen envelope found in ZZ Ceti stars. Recently, Wesemael, Lamontagne, and Fontaine (1986) and Lamontagne, Wesemael, and Fontaine (1987) have obtained and compared ultraviolet observations of several DA white dwarfs, in or near the instability strip, with published model calculations from Nelan and Wegner (1985), hereafter NW, and Koester et al. (1985), hereafter KWZV. They determined the boundaries of the variability region at 11400 - 12500 K or 11700 - 13000 K depending on which grid was used. We present here a reanalysis of these IUE observations with an improved grid of model atmospheres in order to define more precisely the location of the blue edge.
Hot DQ white dwarfs: a pulsational test of the mixing scenario for their formation
