We establish a linear relation between K − and the gas mass-loss rate. Our results show that Tc-poor Miras are redder than Tc-rich Miras in a broad range of the mid-IR, suggesting that the previous finding based on the K − colour is not due to a specific dust feature in the 22 μm band. Literature data of gas mass-loss rates of Miras and semi-regular variables were collected and analysed. Near- and mid-IR photometry and ISO dust spectra of our stars were investigated where available. We analysed new optical spectra and expanded the sample by including more stars from the literature. The most important aim is to investigate if the same two sequences can be revealed in the gas mass-loss rate. Here, the previous sample is extended and the analysis is expanded towards other colours and dust spectra. higher dust mass-loss rates) than Tc-rich Miras at a given period. Contrary to what might be expected, Tc-poor Miras show redder K − colours (i.e. A near- to mid-infrared colour such as K − is a good probe for the dust mass-loss rate of the stars. We follow-up on a previous finding that AGB Mira variables containing the third dredge-up indicator technetium (Tc) in their atmosphere form a different sequence of K − colour as a function of pulsation period than Miras without Tc.
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