When low- and intermediate-mass stars pass through the Asymptotic Giant Branch (AGB) they experience dramatic changes in their circumstellar shell (CSE) influenced by their mass loss, the possible presence of a (closeby) companion and the magnetic field. Masers, well spread in this environment, provide a powerful tool to reveal the CSE changes occurring when the stars undergo a transitional phase on the AGB. These can be indirect, via for instance the modification of the pumping conditions or a direct consequence of e.g. a companion and/or of the magnetic field. Evidences of such changes have been observed towards Miras, materialized by strong - both in intensity and degree of polarisation - (OH) flaring events and towards stars believed to be transitioning from the Mira to the OH/IR phase, showing an unusual high degree of polarisation. How OH maser emission can be used as a signpost of transitional phases along the AGB is explored.

Based on the results of an on-going monitoring program of 1612 MHz OH masers in OH/IR stars, we determined a lifetime encompassing late AGB and early post-AGB evolution of at least 4500 years. Fading of the OH masers observed with the Nançay Radio Telescope is detected in several post-AGB OH/IR stars on timescales of decades, while AllWISE/NEOWISE light curves taken almost in parallel show diverse behaviours.