A SURVEY OF MAIN-LINE OH MASER EMISSION FROM SEMIREGULAR VARIABLES

We have made a high sensitivity (similar to 0.15 Jy) search in the OH main-lines for maser emission from a sample of 181 semiregular variabl es. OH emission was observed towards five stars. Two new OH lines were detected and one tentatively. About 10% of semiregulars with distance s less than 300 pc have associated OH maser emission. The OH semiregul ars show much redder near and far infrared colors than similar non-OH stars. On the average, their mass loss rates and OH luminosities are s lightly lower than in the long period Mira variables. Basing on the op tical and infrared properties of the semiregulars and long period vari ables we confirmed that properties of the SRa stars are intermediate b etween those of SRb and Mim variables. The occurrence of strongly pola rized, low luminosity OH main-line maser only, together with relativel y low efficiency of pumping by the infrared photons, provides evidence that the semiregulars are less evolved than the Mire-type variables. This can support the existence of an evolutionary sequence of SRb-SRa- Mira variables. However, the expansion velocities of some OH semiregul ars are higher than those observed in most type I OH Miras and their p ulsational properties may suggest that they are in a peculiar evolutio nary phase.