Late-type stars of mass less than several solar masses evolve into planetary nebulae (PNe), after the AGB phase of stellar evolution.  During the AGB phase, these stars undergo a period of intensive mass loss.  The short evolutionary phase between the AGB phase and the PN stage is called a proto-planetary nebula (PPN).  Molecular maser emission occurs in the circumstellar shell during the AGB phase and is also found in some PPNe.

An example of a PPN is the interesting object AFGL2343, also known as IRAS19114+002.  It is associated with a bright visible star (V=8) of spectral type G5Ia (HD179821).  OH masers at 1612 and 1667 MHz have been observed with the VLA.  A velocity averaged image of both the 1612 and 1667 MHz emission is shown below.  No 1665 MHz emission has ever been detected, although it has been searched for.