  The properties of carbon stars in the Magellanic Clouds (MCs) and their total
dust production rates are predicted by fitting their spectral energy
distributions (SED) over pre-computed grids of spectra reprocessed by dust. The
grids are calculated as a function of the stellar parameters by consistently
following the growth for several dust species in their circumstellar envelopes,
coupled with a stationary wind. Dust radiative transfer is computed taking as
input the results of the dust growth calculations. The optical constants for
amorphous carbon are selected in order to reproduce different observations in
the infrared and optical bands of \textit{Gaia} Data Release 2. We find a tail
of extreme mass-losing carbon stars in the Large Magellanic Cloud (LMC) with
low gas-to-dust ratios that is not present in the Small Magellanic Cloud (SMC).
Typical gas-to-dust ratios are around $700$ for the extreme stars, but they can
be down to $\sim160$--$200$ and $\sim100$ for a few sources in the SMC and in
the LMC, respectively. The total dust production rate for the carbon star
population is $\sim 1.77\pm 0.45\times10^{-5}$~M$_\odot$~yr$^{-1}$, for the
LMC, and $\sim 2.52\pm 0.96 \times 10^{-6}$~M$_\odot$~yr$^{-1}$, for the SMC.
The extreme carbon stars observed with the Atacama Large Millimeter Array and
their wind speed are studied in detail. For the most dust-obscured star in this
sample the estimated mass-loss rate is $\sim 6.3 \times
10^{-5}$~M$_\odot$~yr$^{-1}$. The grids of spectra are available at:
https://ambrananni085.wixsite.com/ambrananni/online-data-1 and included in the
SED-fitting python package for fitting evolved stars
https://github.com/s-goldman/Dusty-Evolved-Star-Kit .
