¹ Astronomical Institute, Charles University Prague, V Holesovickach 2, CZ-180 00 Praha 8, Bohemia - Czech Republic
² Astronomical Institute, Academy of Sciences of the Czech Republic, CZ-251 65 Ondrejov, Bohemia - Czech Republic

 Cas (HD 5394) is an important representative of Be stars, hot B stars exhibiting Balmer and other emission lines in their spectra at certain epochs. Its emission spectrum was observed as early as in 1863 and has undergone spectacular changes since then.  Cas is also extremely interesting because it qualifies for several phenomenologically defined groups: it is a light and colour variable, long-term radial-velocity variable, X-ray source, IRAS source, rapid line-profile variable, a single-line spectroscopic binary and a central star of a reflection nebula.

A critical compilation of the wealth of observational data on the star, with emphasis on its time variability on several time scales, as well as an estimate of the most probable values of all basic physical properties of the object is presented. It is pointed out that in spite of a large quantity of data, systematic and well-calibrated observations are still rather rare. The present understanding of the object is put into perspective of the more general -- and as yet unsolved -- problem of the formation of Be envelopes.

in IAU Colloquium 187: Exotic Stars as Challenges to Evolution, ed. Tout, C.A. and Van Hamme, W., ASP Conference Series 279, 221
Preprints from hec@sunstel.asu.cas.cz
or on the web at ftp://astro.troja.mff.cuni.cz/hec/gcas2002.ps