A sample of 1497 carbon stars in the Large Magellanic Cloud has been observed
in the red part of the spectrum with the 2dF facility on the AAT. Of these, 156
have been identified as J-type (i.e. 13C-rich) carbon stars using a technique
which provides a clear distinction between J stars and the normal N-type carbon
stars that comprise the bulk of the sample, and yields few borderline cases. A
simple 2-D classification of the spectra, based on their spectral slopes in
different wavelength regions, has been constructed and found to be related to
the more conventional c- and j-indices, modified to suit the spectral regions
observed. Most of the J stars form a photometric sequence in the K - (J-K)
colour magnitude diagram, parallel to and 0.6 mag fainter than the N star
sequence. A subset of the J stars (about 13 per cent) are brighter than this J
star sequence; most of these are spectroscopically different from the other J
stars. The bright J stars have stronger CN bands than the other J stars and are
found strongly concentrated in the central regions of the LMC. Most of the
rather few stars in common with Hartwick and Cowley's sample of suspected CH
stars are J stars. Overall, the proportion of carbon stars identified as J
stars is somewhat lower than has been found in the Galaxy. The Na D lines are
weaker in the LMC J stars than in either the Galactic J stars or the LMC N
stars, and do not seem to depend on temperature.Comment: 19 pages, 21 figures, Latex; in press, MNRA