  The OH main line masers in circumstellar shells were observed with
astrometric VLBI in order to accurately determine the position of the star, as
well as its motion, including the parallax. Results are presented on 4 Mira
stars monitored for 2 - 8 years with the VLBA. The data show that in some stars
the VLBI detection is dominated by blue-shifted emission that is associated
with the stellar image amplified by the maser shell in front of the star. In
other cases the maser is not directly tied to the stellar position, but still
persistent enough to measure proper motion and parallax.
