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 \pm 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 \pm 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 \pm 0.41 mas , also statistically concordant with the VLBI result at the \sim 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 .