  High-resolution spectra of the hot white dwarf G191-B2B, covering the
wavelength region 905-1187A, were obtained with the Far Ultraviolet
Spectroscopic Explorer (FUSE). This data was used in conjunction with existing
high-resolution Hubble Space Telescope STIS observations to evaluate the total
HI, DI, OI and NI column densities along the line of sight. Previous
determinations of N(DI) based upon GHRS and STIS observations were
controversial due to the saturated strength of the DI Lyman-alpha line. In the
present analysis the column density of DI has been measured using only the
unsaturated Lyman-beta and Lyman-gamma lines observed by FUSE. A careful
inspection of possible systematic uncertainties tied to the modeling of the
stellar continuum or to the uncertainties in the FUSE instrumental
characteristics has been performed. The column densities derived are: log N(DI)
= 13.40 +/-0.07, log N(OI) = 14.86 +/-0.07, and log N(NI) = 13.87 +/-0.07
quoted with 2-sigma uncertainties. The measurement of the HI column density by
profile fitting of the Lyman-alpha line has been found to be unsecure. If
additional weak hot interstellar components are added to the three detected
clouds along the line of sight, the HI column density can be reduced quite
significantly, even though the signal-to-noise ratio and spectral resolution at
Lyman-alpha are excellent. The new estimate of N(HI) toward G191-B2B reads: log
N(HI) = 18.18 +/-0.18 (2-sigma uncertainty), so that the average (D/H) ratio on
the line of sight is: (D/H) = 1.66 (+0.9/-0.6) *10^-5 (2-sigma uncertainty).
