CO microwave emission from RV Tau variables.

The detection of CO J = 2-1 and J = 1-0 emission in two RV Tau variables, R Sct and AC Her, and upper limits in other two, U Mon and RV Tau is reported. The detected CO lines are relatively strong, implying the presence of thick envelopes and confirming the post-AGB nature of RV Tau stars. The envelope expansion velocities are low, 2.5-5 km/s. For the observed objects, the CO and far-IR intensities seem to be completely independent magnitudes. This surprising result is very difficult to explain, unless it is assumed that the CO molecules in the RV Tau envelopes are being efficiently photodissociated by the interstellar UV. From the necessary conditions for such a phenomenon, it is determined that a typical time of 2000-3000 years has elapsed from the quenching of the AGB mass-loss phase. The possible future evolution is briefly discussed.