  We report the detection of highly-ionized gas at z~0 seen in resonant UV and
X-ray absorption lines toward the z=0.03 blazar Mkn 421. A total of 13 X-ray
and 3 UV lines were measured (or upper limits derived), including three lines
in the OVII K-series and K\alpha transitions from neon, carbon, and nitrogen.
  From the three OVII lines we derive a 2\sigma Doppler parameter constraint of
24<b<55 km/s. The FUSE spectrum shows strong Galactic low--velocity OVI 1032A
absorption as well as a possible weak OVI high-velocity component (HVC). The
Doppler parameter of the low-velocity OVI measured with FUSE is ~3\sigma higher
than that derived from the OVII line ratios, indicating that the OVII and
Galactic OVI arise in different phases. This velocity dispersion, along with
limits on the gas temperature and density from the X-ray line ratios (assuming
a single phase with collisional ionization equilibrium plus photoionization)
are all consistent with an extragalactic absorber. However, the OVII Doppler
parameter is inconsistent with the high temperature required to produce the
observed OVI_HVC/OVII ratio, implying that the HVC is probably not related to
the OVII. In addition, the OVI_K\alpha line detected by Chandra implies a
column density ~4 times higher than the 1032A absorption. Although an
extragalactic absorber is fully consistent with the measured column density
ratios, a Galactic origin cannot be ruled out given the uncertainties in the
available data.
