A SEARCH FOR WATER AND MAINLINE MASERS IN OH IR STAR MIMICS WITH STRONG SILICATE EMISSION FEATURES/

IRAS sources with a strong 9.7-mu m silicate emission feature have a l ow-resolution spectral type of 28 or 29, and appear to be associated w ith oxygen-rich circumstellar shells. Yet approximate to 40 per cent o f those searched at Arecibo for 1612-MHz emission have none, despite r ed IR colours, (25 - 12) mu m > - 0.55, that point to a thick shell. W e report on a water and mainline OH maser search of a complete sample of these OH/IR star colour mimics, where we find seven new water and f ive new mainline masers. In aggregate, just 10/60 (17 per cent) of thi s sample are without any kind of detected maser. The frequency of wate r and mainline masers in these mimics is about half that in OH/IR star s with the same colour, a result that independently confirms that the shells of mimics are in some way different from those of OH/IR stars. Combining these results with other work on the incidence of masers in O-rich shells, we suggest that the percentage of maserless shells incr eases from approximate to 11 to approximate to 25 per cent at (25 - 12 ) mu m = -0.5.