  We present Very Long Baseline Array observations of the kinematics of the
water masers associated with OH 12.8-0.9, the fourth member of the so-called
"water fountain" class of sources. We find that the masers occupy two distinct
regions at the ends of a bipolar jet-like structure oriented north-south, with
the blue-shifted masers located to the north and the red-shifted masers to the
south. The masers are distributed along arc-like structures 12-20 mas across
oriented perpendicular to the separation axis with an angular separation of
~110 mas on the sky. Our multi-epoch observations, show the two maser arcs to
be expanding away from each other along the axis of separation. The relative
proper motions of the two maser regions is 2.7 mas/yr (~105 km/s at the assumed
distance of 8 kpc). The measured radial velocity difference between the
northern, blue-shifted masers and the southern, red-shifted masers is 48.4
km/s. The radial velocity, when combined with the proper motion, yields a
three-dimensional expansion velocity of 58 km/s and an inclination angle of 24
deg. for the jet. By combining our radial velocities with historical values, we
estimate the three dimensional acceleration of the masers to be ~0.63 km/s/yr
and a dynamical age for the collimated outflow of ~90 yr.
