SEMIREGULAR VARIABLES OF TYPES SRA AND SRB - CIRCUMSTELLAR CO EMISSION OF AN OXYGEN-RICH SAMPLE

O-rich Semiregular Variables (SRVs) of types SRa and SRb have been obs erved in the (CO)-C-12 (J=1-0) and (J=2-1) lines using the SEST, La Si lla, Chile, and the 20-m telescope at Onsala, Sweden. In total 22 dete ctions (13 are new ones) and 1 tentative detection can be reported amo ng the 48 stars observed. In 7 cases detections are precluded because of strong interference from interstellar CO emission. The majority of the detected objects, covering both small and longer periods, are weak in CO, i.e. they are low mass-loss rate objects (less than or equal t o 10(-7) M./yr), and have, with only a few exceptions, envelopes with small expansion velocities (the mean value is approximate to 8 km/s). However, in this respect their properties are very similar to those of bright O-rich Mira variables, whose pulsational periods are on the av erage 2-3 times higher. A comparison between stellar and circumstellar properties shows that the gas expansion velocity does not depend on t he stellar effective temperature, nor on the period. Likewise, we find no correlation between mass-loss rate and period, but there might be a weak dependence of the former on the stellar effective temperature.