Unidentified Features in the Ultraviolet Spectrum of X Per
  High-resolution ultraviolet spectra from the Space Telescope Imaging
Spectrograph (STIS) were used to search for unidentified interstellar
absorption features in the well studied sightline towards X Per (HD 24534). The
significance of features detected was determined from Gaussian fits to the
data, as well as the features' persistence in multiple observations. Fixed
pattern noise characteristics were studied in STIS echelle data to distinguish
between interstellar and instrumental features. We report the detection of two
unidentified features that stand out from the more common fixed pattern noise
features. Both features have depths of > 3% of the continuum level making them
very likely of interstellar origin. Lastly, we comment on possible carriers,
and discuss future prospects for studying these and perhaps other unidentified
lines in larger samples of sightlines.
