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We report observations by the Extreme Ultraviolet Explorer (EUVE) of the spectrum of the hot DA white dwarf G191-B2B. G191-B2B was detected in the medium and long wavelength channels of the spectrometer and exhibits a rich complex of absorption lines, indicating the presence of traces of heavy elements in its otherwise hydrogen-rich photosphere. We have accounted for a number of instrumental effects, including fixed pattern noise and the presence of overlapping spectral orders. The absorption edge of interstellar HeI at 504~\AA\ is clearly detected in the long wavelength (LW) spectrum. We report here a preliminary analysis of the LW spectrum of G191-B2B with a grid of synthetic spectra incorporating new opacities of iron computed by the Opacity Project. As a first step in the analysis of the G191-B2B spectrum, we have determined the interstellar columns of neutral hydrogen and neutral helium in the line of sight of this star. The measured column for HI is $2.0\pm0.2\times 10^{18}$ cm$^{-2}$ and the measured HeI/HI is $0.07\pm0.01$ for effective temperatures of 55000-60000~K and log g = 7.5. This preliminary analysis indicates a slightly higher value for the ISM column than that reported by HUT. Our results implies a neutral hydrogen to neutral helium ratio of $\sim$14, favoring higher ionization of helium in the local interstellar medium if we assume a cosmic abundance ratio of 10 for this ratio.
This work has been supported by NASA's EUVE Guest Observer Program.
