  We present ESO/VLT spectra in the 2.9--4.1 micron range for a large sample of
infrared stars in the Large Magellanic Cloud (LMC), selected on the basis of
MSX and 2MASS colours to be extremely dust-enshrouded AGB star candidates. Out
of 30 targets, 28 are positively identified as carbon stars, significantly
adding to the known population of optically invisible carbon stars in the LMC.
We also present spectra for six IR-bright stars in or near three clusters in
the LMC, identifying four of them as carbon stars and two as oxygen-rich
supergiants. We analyse the molecular bands of C2H2 at 3.1 and 3.8 micron, HCN
at 3.57 micron, and sharp absorption features in the 3.70--3.78 micron region
that we attribute to C2H2. There is evidence for a generally high abundance of
C2H2 in LMC carbon stars, suggestive of high carbon-to-oxygen abundance ratios
at the low metallicity in the LMC. The low initial metallicity is also likely
to have resulted in less abundant HCN and CS. The sample of IR carbon stars
exhibits a range in C2H2:HCN abundance ratio. We do not find strong
correlations between the properties of the molecular atmosphere and
circumstellar dust envelope, but the observed differences in the strengths and
shapes of the absorption bands can be explained by differences in excitation
temperature. High mass-loss rates and strong pulsation would then be seen to be
associated with a large scale height of the molecular atmosphere.
