  The main-line OH masers around 4 AGB stars have been observed with the NRAO
Very Long Baseline Array (VLBA) at 8 epochs over a period of 2.5 years. Using a
phase referencing technique, the position of the most compact maser spot of
each star was monitored with respect to two extragalactic reference sources.
For U Her and W Hya, we observe the most blue-shifted maser spot, while for R
Cas and S CrB we only detect a compact red-shifted maser spot. We managed to
determine an accurate proper motion and parallax for U Her, R Cas and S CrB,
while additional motion of the compact blue-shifted maser of W Hya is shown to
possibly be related to the stellar pulsation. The motion and radio position are
compared with the stellar trajectory and absolute optical position determined
by the Hipparcos satellite. For U Her and W Hya, the most blue-shifted maser is
consistent with the amplified stellar image. The new distances are compared
with several published P--L relations, and in this respect the VLBI distances
seem an improvement upon the Hipparcos distances.
