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We present the principal astrometric results of the very-long-baselineinterferometry (VLBI) program undertaken in support of the Gravity Probe B(GP-B) relativity mission. VLBI observations of the GP-B guide star, the RS CVnbinary IM Pegasi (HR 8703), yielded positions at 35 epochs between 1997 and2005. We discuss the statistical assumptions behind these results and ourmethods for estimating the systematic errors. We find the proper motion of IMPeg in an extragalactic reference frame closely related to the InternationalCelestial Reference Frame 2 (ICRF2) to be -20.83 +- 0.03 +- 0.09 mas/yr inright ascension and -27.27 +- 0.03 +- 0.09 mas/yr in declination. For eachcomponent the first uncertainty is the statistical standard error and thesecond is the total standard error (SE) including plausible systematic errors.We also obtain a parallax of 10.37 +- 0.07 mas (distance: 96.4 +- 0.7 pc), forwhich there is no evidence of any significant contribution of systematic error.Our parameter estimates for the ~25-day-period orbital motion of the stellarradio emission have SEs corresponding to ~0.10 mas on the sky in eachcoordinate. The total SE of our estimate of IM Peg's proper motion is ~30%smaller than the accuracy goal set by the GP-B project before launch: 0.14mas/yr for each coordinate of IM Peg's proper motion. Our results ensure thatthe uncertainty in IM Peg's proper motion makes only a very small contributionto the uncertainty of the GP-B relativity tests.

VLBI for Gravity Probe B. V. Proper Motion and Parallax of the Guide Star, IM Pegasi