  We present the results of a long (~93 ksec) XMM-Newton observation of the
bright BL-Lac object PKS 0548-322 (z=0.069). Our Reflection Grating
Spectrometer (RGS) spectrum shows a single absorption feature at an observed
wavelength lambda=23.33 A, which we interpret as OVI Ka absorption at z=0.058,
i.e., ~3000 km/s from the background object. The observed equivalent width of
the absorption line ~30 mA, coupled with the lack of the corresponding
absorption edge in the EPIC pn data, implies an OVI column density ~ 2 10^{16}
cm-2 and turbulence with a Doppler velocity parameter b>100 km/s. Within the
limitations of our RGS spectrum, no OVII or OV Ka absorption are detected.
Under the assumption of ionisation equilibrium by both collisions and the
extragalactic background, this is only marginally consistent if the gas
temperature is ~250,000 K, with significantly lower or higher values being
excluded by our limits on OV or OVII. If confirmed, this would be the first
X-ray detection of a large amount of intervening warm absorbing gas through OVI
absorption. The existence of such a high column density absorber, much stronger
than any previously detected one in OVI, would place stringent constraints on
the large-scale distribution of baryonic gas in the Universe.
