(Sub-)Stellar Spectral Sequences

Sandy Leggett
Joint Astronomy Centre

In the last three years astronomers have had to add two new
spectral classes to describe objects cooler than "M".  I will
review the discovery of this very low-mass population and
present infrared spectroscopic data showing new observational
sequences, with particular emphasis on the onset of methane
absorption.  The effective temperatures for the two classes
are typically ~2000 K  and ~1000 K; some of the difficulties
modelling these very cool atmospheres are described.  The best
current estimate of the range in values of temperature and mass
for this population is given