  We present a study of the deuterium abundance along the extended sight line
(2.7kpc) toward HD 90087 with FUSE. Both in terms of distance and column
densities, HD 90087 has the longest and densest sight line observed in the
Galactic disk for which a deuterium abundance has been measured from UV
absorption lines. Because many interstellar clouds are probed along this sight
line, possible variations in the properties of individual clouds should be
averaged out. This would yield a deuterium abundance which is characteristic of
the interstellar medium on scales larger than the Local Bubble. We report
D/O=(1.7+/-0.7)e-2 and D/H=(9.8+/-3.8)e-6 (2 sigma). Our new results confirm
that the gas-phase deuterium abundance in the distant interstellar medium is
significantly lower than the one measured within the Local Bubble. We
supplement our study with a revision of the oxygen abundance toward the
moderately distant star Feige 110 (~200 pc). Excluding saturated lines from the
fits of the FUSE spectra is critical; this led us to derive an OI column
density about two times larger than the one previously reported for Feige 110.
The corresponding updated D/O ratio on this sight line is D/O=(2.6+/-1.0)e-2 (2
sigma), which is lower than the one measured within the Local Bubble. The
dataset available now outside the Local Bubble shows a contrast between the
constancy of D/O and the variability of D/H. As oxygen is considered to be a
good proxy for hydrogen within the interstellar medium, this discrepancy is
puzzling. (abstract abridged)
