Resumen
That strong jets can be launched at very early stages in preplanetary nebulae is nowhere better evidenced than in the class of ``water fountain nebulae'': systems in which very high-velocity H[2]O masers occur along the polar axis. The most extreme member known in this class is IRAS 16342-3814, which displays H[2]O maser emission at radial velocities of +/-160km/s relative to the central, systemic velocity. It thus places the strongest constraints on the timing and kinematics of the polar outflows in such systems. Here, we describe this class of object, and report on the first results from our VLBA study of water masers in IRAS 16342. In this ongoing study, the proper motions of the masers, when combined with their radial velocities, should eventually provide a rather complete picture of the evolution of the jet dynamics.
