  We present ultraviolet spectra of Mira's companion star from the Space
Telescope Imaging Spectrograph (STIS) instrument on board the Hubble Space
Telescope (HST). The companion is generally assumed to be a white dwarf
surrounded by an accretion disk fed by Mira's wind, which dominates the UV
emission from the system. The STIS UV spectrum is dominated by numerous, narrow
H2 lines fluoresced by H I Ly-alpha, which were not detected in any of the
numerous observations of Mira B by the International Ultraviolet Explorer
(IUE). The high temperature lines detected by IUE (e.g., C IV 1550) still exist
in the STIS spectrum but with dramatically lower fluxes. The continuum fluxes
in the STIS spectra are also much lower, being more than an order of magnitude
lower than ever observed by IUE, and also an order of magnitude lower than
fluxes observed in more recent HST Faint Object Camera objective prism spectra
from 1995. Thus, the accretion rate onto Mira B was apparently much lower when
STIS observed the star, and this change altered the character of Mira B's UV
spectrum.
