  Aims: Accurate distances to evolved stars with high mass loss rates are
needed for studies of many of their fundamental properties. However, as these
stars are heavily obscured and variable, optical and infrared astrometry is
unable to provide enough accuracy.
  Methods: Astrometry using masers in the circumstellar envelopes can be used
to overcome this problem. We have observed the OH masers of a number of
Asymptotic Giant Branch (AGB) stars for approximately 1 year with the Very Long
Baseline Array (VLBA). We have used the technique of phase referencing with
in-beam calibrators to test the improvements this technique can provide to Very
Long Baseline Interferometry (VLBI) OH maser astrometric observations.
  Results: We have significantly improved the parallax and proper motion
measurements of the Mira variable stars U Her, S CrB and RR Aql.
  Conclusions: It is shown that both in-beam phase-referencing and a decrease
in solar activity during the observations significantly improves the accuracy
of the astrometric observations. The improved distances to S CrB (418 +21 -18
pc) and RR Aql (633 +214 -128 pc) are fully consistent with published P-L
relations, but the distance to U Her (266 +32 -28 pc) is significantly smaller.
We conclude that for sources that are bright and have a nearby in-beam
calibrator, VLBI OH maser astrometry can be used to determine distances to OH
masing stars of up to ~2 kpc.
