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Water maser emission in planetary nebulae

Planetary Nebulae (PNe) evolve from Asymptotic Giant Branch (AGB) stars after a brief post-AGB phase. Water maser emission is characteristic of oxygen-rich AGB stars, is observed in post-AGB stars and, unexpectedly, has been detected in three PNe (IRAS17347-3139, IRAS18061-2505 and IRAS19255+2123) where the physical conditions to generate water maser emission did not seem to exist. These three objects may be considered as the youngest PNe known up to date and, therefore, they are key objects to understand the formation of PNe. In addition, the existence of water maser PNe allow us to study every phase in the AGB to PN transition using water maser emission which can be observed at very high spatial and spectral resolution. In this paper we review the properties of water maser emission in PNe, the existing observations of the three water maser emitting PNe and their implications in our understanding of PN formation and evolution.