  The luminosities and periods of obscured AGB stars in the LMC are discussed
using a combination of ISO and ground-based infrared photometry. The bolometric
luminosities of these stars fall close to an extrapolation of the
period-luminosity relation derived for oxygen-rich Mira variables, with both
oxygen- and carbon-rich stars falling close to the same relation. It has been
known for many years that there are, in the Magellanic Clouds, significant
numbers of large amplitude variables which have considerably higher
luminosities than the period-luminosity relation would predict. Many of these
can be shown to be undergoing hot bottom burning. It is speculated that all
large amplitude AGB stars with luminosities significantly higher than indicated
by the period-luminosity relation are undergoing hot bottom burning.
