  SS Cygni is one of the brightest dwarf novae (DNe), and one of the
best-studied prototypes of the cataclysmic variables. Astrometric observations
with the Fine Guidance Sensors (FGS) on the Hubble Space Telescope (HST),
published in 2004, gave an absolute trigonometric parallax of 6.06+/-0.44 mas.
However, recent very-long-baseline interferometry (VLBI), obtained during radio
outbursts of SS Cyg, has yielded a significantly larger absolute parallax of
8.80+/-0.12 mas, as well as a large difference in the direction of the proper
motion compared to the HST result. The VLBI distance reduces the implied
luminosity of SS Cyg by about a factor of two, giving good agreement with
predictions based on accretion-disk theory in order to explain the observed DN
outburst behavior. This discrepancy raises the possibility of significant
systematic errors in FGS parallaxes and proper motions. We have reanalyzed the
archival HST/FGS data, including (1) a critical redetermination of the
parallaxes of the background astrometric reference stars, (2) updated input
values of the reference-star proper motions, and (3) correction of the position
measurements for color-dependent shifts. Our new analysis yields a proper
motion of SS Cyg that agrees well with the VLBI motion, and an absolute
parallax of 8.30+/-0.41 mas, also statistically concordant with the VLBI result
at the ~1.2 sigma level. Our results suggest that HST/FGS parallaxes are free
of large systematic errors, when the data are reduced using high-quality input
values for the astrometry of the reference stars, and when instrumental
signatures are properly removed.
