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Aims: We report the discovery that Mira variables with and without absorptionlines of the element technetium (Tc) occupy two different regions in a diagramof near- to mid-infrared colour versus pulsation period. Tc is an indicator ofa recent or ongoing mixing event called the third dredge-up (3DUP), and thenear- to mid-IR colour, such as the (K-[22]) colour where [22] is the the 22micron band of the WISE space observatory, is an indicator of the dustmass-loss rate of a star. Methods: We collected data from the literature aboutthe Tc content, pulsation period, and near- and mid-infrared magnitudes of morethan 190 variable stars on the asymptotic giant branch (AGB) to which Mirasbelong. The sample is naturally biased towards optical AGB stars, which havelow to intermediate (dust) mass-loss rates. Results: We show that a clearrelation between dust mass-loss rate and pulsation period exists if adistinction is made between Tc-poor and Tc-rich Miras. Surprisingly, at a givenperiod, Tc-poor Miras are redder in (K-[22]) than are Tc-rich Miras; i.e. theyhave higher mass-loss rates than the Tc-rich Miras. A few stars deviate fromthis trend; physical explanations are given for these exceptions, such asbinarity or high mass. Conclusions: We put forward two hypotheses to explainthis dichotomy and conclude that the two sequences formed by Tc-poor andTc-rich Miras are probably due to the different masses of the two groups. Thepulsation period has a strong correlation with the dust-mass loss rate,indicating that the pulsations are indeed triggering a dust-driven wind. Thelocation in the (K-[22]) vs. period diagram can be used to distinguish betweenpre- and post-3DUP Miras, which we apply to a sample of Galactic bulge AGBstars. We find that 3DUP is probably not common in AGB stars in the innerbulge.

Period -- mass-loss rate relation of Miras with and without technetium