  We have obtained Hubble Space Telescope/STIS low-resolution ultraviolet
spectra of the X-ray pulsar 4U 1626-67 (=KZ TrA); 4U 1626-67 is unusual even
among X-ray pulsars due to its ultra-short binary period (P=41.4 min) and
remarkably low mass-function (<1.3e-6 Msun). The far-UV spectrum was exposed
for a total of 32ks and has sufficient signal-to-noise to reveal numerous broad
emission and prominent narrower absorption lines. Most of the absorption lines
are consistent in strength with a purely interstellar origin. However, there is
evidence that both CI and CIV require additional absorbing gas local to the
system. In emission, the usual prominent lines of NV and HeII are absent,
whilst both OIV and OV are relatively strong. We further identify a rarely seen
feature at ~1660A as the OIII] multiplet. Our ultraviolet spectra therefore
provide independent support for the recent suggestion that the mass donor is
the chemically fractionated core of either a C-O-Ne or O-Ne-Mg white dwarf;
this was put forward to explain the results of Chandra high-resolution X-ray
spectroscopy. The velocity profiles of the ultraviolet lines are in all cases
broad and/or flat-topped, or perhaps even double-peaked for the highest
ionization cases of O; in either case the ultraviolet line profiles are in
broad agreement with the Doppler pairs found in the X-ray spectra. Both the
X-ray and far-UV lines are plausibly formed in (or in an corona just above) a
Keplerian accretion disc; the combination of ultraviolet and X-ray spectral
data may provide a rich data set for follow-on detailed models of the disk
dynamics and ionization structure in this highly unusual low-mass X-ray pulsar
system.
