LOW-DENSITY OF NEUTRAL HYDROGEN AND HELIUM IN THE LOCAL INTERSTELLAR-MEDIUM - EXTREME-ULTRAVIOLET EXPLORER PHOTOMETRY OF THE LYMAN CONTINUUM OF THE HOT WHITE-DWARFS MCT 0501-2858, MCT 0455-2812, HZ 43, AND GD 153

The first comprehensive sky survey of the extreme ultraviolet (EUV) sp ectral range performed by the Extreme Ultraviolet Explorer (EUVE) has uncovered a handful of very bright sources at wavelengths longer than the He I 504 Angstrom photoionization edge. Among these objects are fo ur white dwarfs with exceptionally low interstellar medium (ISM) colum n densities along the line of sight. Analysis of EUV photometry of the He-rich DO white dwarf MCT 0501-2858 and the H-rich DA white dwarf MC T 0455-2812 along one line of sight and of the DA white dwarfs HZ 43 a nd GD 153 near the north Galactic pole indicates that the overall mini mum column density of the neutral material centered on the Sun is N(H I) = 0.5-1.0 x 10(18) cm(-2) In the case of MCT 0501-2858, EUV photome tric measurements provide a clear constraint to the effective temperat ure (60,000-70,000 K). Given these neutral hydrogen columns, the actua l contribution to the density of neutral species from the immediate so lar environment (the ''local fluff'') would only cover a distance of a pproximate to 2-3 pc (assuming an average density n(H I) = 0.1 cm(-3)) leaving these lines of sight almost entirely within the hot phase of the ISM. A preliminary examination of the complete EUVE long-wavelengt h survey indicates that these lines of sight are exceptional and set a minimum column density in the solar environment.