New detections of H2O masers in planetary nebulae and post-AGB stars using the Robledo-70 m antenna

Aims. We investigated the possible relationship between the evolutionary stage of post-AGB stars and planetary nebulae (PNe) and the presence of water masers in their envelopes Methods. We have used NASA's 70-m antenna in Robledo de Chavela ( Spain) to search for the water maser transition at 22 235.08 MHz, towards a sample of 105 sources with IRAS colour characteristic of post-AGB stars and PNe at declination >- 32 degrees. 83% of the sources in the sample are post-AGB stars, 15% PNe or PN candidates, while only 2% seem to be HII regions. Results. We have detected five water masers, of which four are reported for the first time: two in PNe ( IRAS 17443-2949 and IRAS 18061- 2505), a "water fountain" in a post-AGB star ( IRAS 16552-3050), and one in a source previously catalogued as a PN, but whose classification is uncertain ( IRAS 17580-3111). Conclusions. The unexpected detections of water masers in two objects among the small subset of PNe led us to suggest that the PNe harbouring water masers are a special type of massive, rapidly evolving PNe.