  The luminous blue variable (LBV) RMC143 is located in the outskirts of the
30~Doradus complex, a region rich with interstellar material and hot luminous
stars. We report the $3\sigma$ sub-millimetre detection of its circumstellar
nebula with ALMA. The observed morphology in the sub-millimetre is different
than previously observed with HST and ATCA in the optical and centimetre
wavelength regimes. The spectral energy distribution (SED) of RMC143 suggests
that two emission mechanisms contribute to the sub-mm emission: optically thin
bremsstrahlung and dust. Both the extinction map and the SED are consistent
with a dusty massive nebula with a dust mass of $0.055\pm0.018~M_{\odot}$
(assuming $\kappa_{850}=1.7\rm\,cm^{2}\,g^{-1}$). To date, RMC143 has the most
dusty LBV nebula observed in the Magellanic Clouds. We have also re-examined
the LBV classification of RMC143 based on VLT/X-shooter spectra obtained in
2015/16 and a review of the publication record. The radiative transfer code
CMFGEN is used to derive its fundamental stellar parameters. We find an
effective temperature of $\sim 8500$~K, luminosity of log$(L/L_{\odot}) =
5.32$, and a relatively high mass-loss rate of $1.0 \times
10^{-5}~M_{\odot}$~yr$^{-1}$. The luminosity is much lower than previously
thought, which implies that the current stellar mass of $\sim8~M_{\odot}$ is
comparable to its nebular mass of $\sim 5.5~M_{\odot}$ (from an assumed
gas-to-dust ratio of 100), suggesting that the star has lost a large fraction
of its initial mass in past LBV eruptions or binary interactions. While the
star may have been hotter in the past, it is currently not hot enough to ionize
its circumstellar nebula. We propose that the nebula is ionized externally by
the hot stars in the 30~Doradus star-forming region.
