A survey of main-line OH maser emission from semiregular variables.

We have made a high sensitivity (~0.15Jy) search in the OH main-lines for maser emission from a sample of 181 semiregular variables. OH emission was observed towards five stars. Two new OH lines were detected and one tentatively. About 10% of semiregulars with distances less than 300pc have associated OH maser emission. The OH semiregulars show much redder near and far infrared colors than similar non-OH stars. On the average, their mass loss rates and OH luminosities are slightly lower than in the long period Mira variables. Basing on the optical and infrared properties of the semiregulars and long period variables we confirmed that properties of the SRa stars are intermediate between those of SRb and Mira variables. The occurrence of strongly polarized, low luminosity OH main-line maser only, together with relatively low efficiency of pumping by the infrared photons, provides evidence that the semiregulars are less evolved than the Mira-type variables. This can support the existence of an evolutionary sequence of SRb-SRa-Mira variables. However, the expansion velocities of some OH semiregulars are higher than those observed in most type I OH Miras and their pulsational properties may suggest that they are in a peculiar evolutionary phase.