  We reinvestigate the question of spatial variation of the local D/H
abundance, using both archival GHRS spectra, and new echelle spectra of
G191-B2B obtained with the Space Telescope Imaging Spectrograph (STIS) aboard
HST. Our analysis uses stratified line-blanketed non-LTE model atmosphere
calculations to determine the shape of the intrinsic WD Lyman-alpha profile and
estimate the WD photospheric contamination of the interstellar lines. Although
three velocity components were reported previously towards G191-B2B, we detect
only two velocity components. The first component is at V(hel) ~ 8.6 km/s and
the second at V(hel) ~ 19.3 km/s, which we identify with the Local Interstellar
Cloud (LIC). From the STIS data we derive D/H = 1.60(+0.39,-0.27)X10^-5 for the
LIC component, and D/H > 1.26X10^-5 for the 8.6 km/s component (uncertainties
denote 2-sigma or 95% confidence limits). The STIS data provide no evidence for
local or component-to-component variation in the D/H ratio. Despite using two
velocity components for the profile fitting and using a more physically
realistic WD Lyman-alpha profile for G191-B2B, our re-analysis of the GHRS data
indicates a component-to-component variation as well as a variation of the D/H
ratio in the LISM, neither of which are supported by the newer STIS data. We
believe the most probable cause for this difference is the characterization of
the background due to scattered light in the GHRS and STIS spectrographs. The
two-dimensional MAMA detectors of STIS measure both the spatial and wavelength
dependences of scattered light, allowing more accurate scattered light
corrections than was possible with GHRS.
