S Doradus variables in the Galaxy and the Magellanic Clouds

The goal in writing this paper is five fold: (1) to summarize the scientifi c achievements in the 20th century on S Dor variables (or LBVs); (2) to pre sent an inventory of these variables: in the Galaxy and the Magellanic Clou ds with a description of their physical state and instability properties; ( 3) to emphasize the photometric achievements of the various types of instab ilities. Generally this seems to be a neglected item resulting in a number of misunderstandings continuously wandering through literature; (4) to inve stigate the structure of the S Dor-area on the HR-diagram; (5) to estimate the total numbers of S Dor variables in the three stellar systems. The posi tion of the strong active S Dor variables in minimum brightness obey the fo llowing: linear relation on the HR-diagram:log L/L. = 1.37 log T-eff - 0.03 . The relatively small dispersion of less active and supposed ex-and dorman t S Dor variables with respect to this relation is twice as large at the bl ue side than at the red side. This might be caused by evolution to the WR s tage and/or to high rotation. S Dor variables carl be subject to five types of instabilities: the very rare genuine eruptive episodes (the "SD-eruptio ns"), two different brightening phases caused by slow pulsations (the "SD-p hases"): one on a time scale of years, the other on a time scale of decades at a more or less constant luminosity and two types of microvariations: on e on a time scale of weeks, the other on a time scale of about 100 d. So fa r, no periodicities of light curve characteristics of ally of these instabi lities have ever been found. The durations of active and non-active stages are estimated fur about half of the sample based on scattered magnitude est imations such as from historical records, and on modern monitoring campaign s. It would be a misunderstanding to believe that all S Dor variables shoul d be always spectacular. It is estimated that most of them will not. be spe ctacular at all for at least 70% of their lifetime as an S Dor variable.