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Water fountain stars represent a stage between the asymptotic giant branch(AGB) and planetary nebulae phases, when the mass loss changes from sphericalto bipolar. These types of evolved objects are characterized by high-velocityjets in the 22 GHz water maser emission. We surveyed the CO and 13CO lineemission towards a sample of ten water fountain stars through observing theJ=1-0 and 2-1 lines of CO and 13CO, using the 30m IRAM radio telescope at PicoVeleta. All the water fountains visible from the observatory were surveyed.Most of the line emission arises from foreground or background Galactic clouds,and we had to thoroughly analyse the spectra to unveil the velocity componentsrelated to the stars. In two sources, IRAS 18460-0151 and IRAS 18596+0315, weidentified wide velocity components with a width of 35-40 km/s that are centredat the stellar velocities. These wide components can be associated with theformer AGB envelope of the progenitor star. A third case, IRAS 18286-0959, isreported as tentative; in this case a pair of narrow velocity components,symmetrically located with respect to the stellar velocity, has beendiscovered. We also modelled the line emission using an LVG code and derivedsome global physical parameters, which allowed us to discuss the possibleorigin of this gas in relation to the known bipolar outflows. For IRAS18460-0151 and IRAS 18596+0315, we derived molecular masses close to 0.2 solarmasses, mean densities of 10^4 cm^{-3}, and mass-loss rates of 10^{-4}M_sun/yr. The kinetic temperatures are rather low, between 10 and 50 K in bothcases, which suggests that the CO emission is arising from the outer and coolerregions of the envelopes. No fitting was possible for IRAS 18286-0959, becauseline contamination can not be discarded in this case.

Sensitive CO and 13CO survey of water fountain stars. Detections towards IRAS 18460-0151 and IRAS 18596+0315