A shock-excited OH maser in a post-asymptotic giant branch envelope?

We have observed a sample of OH 1612 MHz masing objects in all four OH grou nd-state transitions with the Australia Telescope Compact Array. One likely post-asymptotic giant branch (AGB) object is found to emit in the 1612, 16 65, and 1720 MHz transitions. We discuss the evidence that this object may be an early post-AGB object and the possibility for such a circumstellar en velope to harbor a 1720 MHz maser. We argue that during a very brief period , just after the star has left the thermally pulsing phase of the AGB and t he wind velocity starts to increase, post-AGB objects might show 1720 MHz e mission. The best objects to search for such emission would be those that a re masing at 1612 and 1665 MHz, but not at 1667 MHz nor in the 22 GHz H2O t ransition.