The OH 1667 MHz maser in the circumstellar shell around the Mira variable U Her has been observed with the NRAO Very Long Baseline Array ( VLBA ) at 6 epochs , spread over 4 years .
Using phase referencing techniques the position of the most blue-shifted maser spot was monitored with respect to two extra-galactic radio sources .
The absolute radio positions of the maser can be compared with the stellar optical position measured by the Hipparcos satellite to 15 mas accuracy .
This confirms the model in which one of the maser spots corresponds to the stellar continuum , amplified by the maser .
The stellar proper motion and the annual parallax ( \pi _ { VLBI } = 5.3 \pm 2.1 mas ) were measured .