2004MNRAS.347.1084S

Irregular variables of type Lb share many characteristics with asymptotic giant branch (AGB) stars such as Miras and semiregulars (SRs). Despite their large number, irregulars are poorly understood. Using the large Nanay radio telescope, we searched for 18-cm OH maser emission from these stars. We selected two samples of Lbs that present a noticeable circumstellar envelope. In the first one, 44 high-latitude objects with infrared colours of dusty mass-losing stars were observed in a wide-bandwidth velocity-search mode. In addition, 16 objects (including six from the first sample), in which other circumstellar molecular lines had recently been found, were observed with higher resolution and sensitivity. We did not detect any new OH maser emission from these samples down to 3σ sensitivity limits ranging from 30 to 200 mJy. We derived upper limits for the efficiency of a typical pump using far-infrared photons. When compared with known OH masers in AGB stars, we show that a significant gap is left unexplored below the threshold that we determined for Lbs. Lower efficiencies exist in some OH Miras and in the OH SRs. We cannot discard the possibility of faint OH emission from certain Lbs, based on the same processes as encountered in those sources. We briefly discuss physical conditions allowing OH emission in the envelope, and the evolutionary stage of objects classified as Lb variables with respect to the AGB.