EX Orionis stars are T Tauri stars showing large scale photometric
 outbursts on time scales of months and years. Two ideas are proposed
 to explain their behavior. The EX Orionis stars could be post
 FU Orionis stars, where the disk has not yet reached a stable
 condition leading to outbreaks, or the outbreaks could be small
 versions of FU Orionis events. In either of the two cases, the
 variations are caused by variations of the accretion rate due to
 instabilities, and the study of the disks of Ex Orionis stars will
 lead to a better understanding of disk instabilities which are
 supposed to be important for the evolution of disk of T Tauri stars.
 Full LWS and SWS spectra are proposed for two reasons:

 1. To test some models directly:
 Changes in the opacities due to the dissociation of certain molecules,
 especially H2_0, or the evaporation of dust grains play a crucial
 role in some outbreak scenarios. The LWS/SWS spectroscopy will allow
 us to detect the most abundant molecules directly and will allow us
 to decide if their dissociation may lead to the outbreaks.

 2. To investigate the properties of their disks, and winds,
 which can than be compared with other T Tauri stars to see
 what the difference is. The strength of emission lines formed in
 the wind can be used to derive mass outflow rates of the wind,
 and to investigate the disk/wind connection. Lines from
 the disk will allow us to detect holes or regions of very high
 density  and temperature in the disk.