2007AJ....133.2679H

EX Lupi is the prototype of the ``EXor'' class, which are pre-main-sequence variables that normally remain at minimum light, but are subject to relatively brief (a few months to a few years) flare-ups of several magnitudes amplitude. This paper examines what is known about EX Lup itself, and describes new photometric and spectroscopic information collected between 1995 and 2005, during which time the star underwent four flare-ups. It is concluded, in agreement with previous investigations, that the flare-ups are due to intermittent mass infall. The evidence: veiling of the M0-type absorption spectrum, appearance of reversed P Cyg-type absorption components displaced up to +340 km/s at many emission lines, and striking variations in emission-line structure. It remains to be seen whether these phenomena are shared by other EXors, which are often classified as such on the basis of fragmentary observational evidence.