  The 22 GHz H_2O masers in the circumstellar envelope of the Mira variable
star U Her have been observed with MERLIN using a phase referencing technique
to determine accurate astrometric positions. The positions were compared with
the optical positions obtained with the Hipparcos satellite to an accuracy of
18 mas. The absolute radio position of the brightest H_2O maser spot is found
to match the optical position, indicating that this spot is the stellar image
amplified by the maser screen in front of it. The occurrence of an amplified
image in the 22 GHz maser can be used to accurately determine the positions of
the H_2O with respect to the star as well as with respect to the SiO and OH
masers. Our observations seem to indicate that the star is not in the centre of
the distribution of maser spots, which has been interpreted as a ring.
