[en] The observed frequency with which infrared excesses appear in F, G, and K supergiants of luminosity class Ia supports the idea that these exesses arise in a ''fossil' circumstellar dust shell that was formed during a prior M-supergiant phase of evolution. The required leftward evolution of the star on the H-R diagram would then imply that the Ledoux, rather than the Schwarzschild, criterion for convective mixing is the correct criterion to use in stellar evolution calculations