  Using Far Ultraviolet Spectroscopic Explorer and Hubble Space Telescope
observations of the QSO PG1259+593, we detect D I Lyman-series absorption in
high velocity cloud Complex C, a low-metallicity gas cloud falling onto the
Milky Way. This is the first detection of atomic deuterium in the local
universe in a location other than the nearby regions of the Galactic disk. A
new Westerbork Synthesis Radio Telescope (WSRT) interferometer map of the H I
21 cm emission toward PG1259+593 indicates that the sight line passes through a
compact concentration of neutral gas in Complex C. We find D/H =
(2.2+/-0.7)x10^-5, O/H = (8.0+/-2.5)x10^-5, and D/O = 0.28+/-0.12. The
metallicity of Complex C gas toward PG1259+593 is approximately 1/6 solar, as
inferred from the oxygen abundance [O/H] = -0.79 (+0.12, -0.16). While we
cannot rule out a value of D/H similar to that found for the local ISM, we can
confidently exclude values as low as those determined recently for extended
sight lines in the Galactic disk. Combined with the sub-solar metallicity
estimate and the low nitrogen abundance, this conclusion lends support to the
hypothesis that Complex C is located outside the Milky Way, rather than inside
in material recirculated between the Galactic disk and halo. The value of D/H
for Complex C is consistent with the primordial abundance of deuterium inferred
from recent Wilkinson Microwave Anisotropy Probe observations of the cosmic
microwave background and simple chemical evolution models that predict the
amount of deuterium astration as a function of metallicity. [Abbreviated
abstract]
