2000A&AS..146..179E

We present here the results and interpretation of a long-term OH variability program conducted with the French Nancay Radiotelescope from 1980 to 1995. It concerns seven Mira stars: R Aql, RS Vir, S CrB, R LMi, RR Aql, U Her and UX Cyg. This study deals with the three OH maser lines observed in the Miras at 1612, 1665 and 1667 MHz. These OH variable stars have periods ranging from 290 to 580 days. The study presents the first insight of the long-term temporal behaviour of OH integrated flux variations as well as spectral component variations. The main aims are to determine the temporal behaviour of the OH maser emission and the longevity and variability of the spectral components. We find that the shapes of the OH curve are closer to the IR than the optical shapes and that the emissions at 1665 and 1667 MHz have a very similar behaviour while the emission at 1612 MHz behaves differently. The 1612 MHz emission shows smoother temporal variations and greater component longevity than the main line emission, leading to the conclusion that the 1612 MHz emission is coming from an outer part of the circumstellar shell and is more saturated than the main line emission. The study also shows the existence of inhomogeneities, especially differences between the front and back parts of the shell can be seen, and that OH variability curves undergo long term variations over several cycles.