2002A&A...388..252E

The 22 GHz water vapor masers of four ``non-variable'' OH/IR stars considered to be young Proto-Planetary Nebulae (PPNs) were monitored for more than 10 years. In two of them, OH15.7+0.8 and OH17.7-2.0 having Mira-like line profiles, the maser flux decreased and the maser finally disappeared. The decrease of the maser brightness is spectacular for OH17.7-2.0 in particular. The maser had a flux density of several hundred Jansky in the eighties and is now absent since 1990. The masers of OH15.7+0.8 and OH17.7-2.0 probably emerged in the ``dying AGB wind'' during the mass loss reduction process accompanying the departure from the AGB. The other two, OH12.8-0.9 and OH37.1-0.8, have double-peaked profiles with a considerably larger velocity spread than the OH masers in these sources. The velocities are incompatible with the spherical symmetric mass loss process on the AGB and give evidence for the presence of probably bipolar outflows with a projected velocity of v_e≃28km/s during the very early PPN phase. The gross structure of their profiles was rather stable during the monitor period while individual maser lines appeared and disappeared. An increase of the velocity range was found in both sources implying that the outflow is accelerating. Analysis of the profile variations of all four objects suggests a lifetime of individual maser components of 1-3 years. There is no evidence for ordered motion traced by the masers. The H₂O maser properties of all these objects are strongly different from those of ordinary OH/IR stars, allowing to find new young PPNs among optically still hidden objects by looking for irregular maser profiles. In contrast to current definitions in the literature ``non-variable'' OH/IR stars are proposed to be classified as (young) PPNs. It is argued that beyond the end of the AGB, maser properties cannot be used to infer the evolutionary stage of the objects in transition to Planetary Nebulae.