Do evolved stars in the LMC show dual dust chemistry?
  We study a group of evolved M-stars in the Large Magellanic Cloud,
characterized by a peculiar spectral energy distribution. While the $9.7~\mu$m
feature arises from silicate particles, the whole infrared data seem to suggest
the presence of an additional featureless dust species. We propose that the
circumstellar envelopes of these sources are characterized by a dual dust
chemistry, with an internal region, harbouring carbonaceous particles, and an
external zone, populated by silicate, iron and alumina dust grains. Based on
the comparison with results from stellar modelling that describe the dust
formation process, we deduce that these stars descend from low-mass ($M <
2~M_{\odot}$) objects, formed $1-4$ Gyr ago, currently evolving either in the
post-AGB phase or through an after-pulse phase, when the shell CNO nuclear
activity is temporarily extinguished. Possible observations able to confirm or
disregard the present hypothesis are discussed.
