  We present the discovery that Mira variables separate in two distinct
sequences in a near- to mid-IR color versus pulsation period diagram, if a
distinction is made with respect to the presence of technetium (Tc) in the
stars. Tc is an indicator of recent or ongoing deep mixing during a third
dredge-up event. At a given period, the Tc-poor Miras are redder in $K-[22]$
(i.e.\ have higher dust mass-loss rate) than the Tc-rich Miras. This is
counter-intuitive since the Tc-rich Miras are expected to be more evolved and
should have a higher mass-loss rate. In this contribution we give an update on
this recently discovered conundrum.
