1997A&A...318...81V

A century of photometric observations of AG Car and S Dor is investigated for general characteristics and possible periodicities in the occurrence of the S Dor (SD) phases, defined as episodes of a radius and an apparent brightness variation at a more or less constant luminosity. We identified two types of such phases: the ``very-long-term" (VLT) and the ``normal" SD phases. The latter are superimposed on the first. The time scales for the VLT-SD phases are in the order of decades for both variables. The normal SD phases of AG Car and presumably also of S Dor obey stable periods of 371.4d ±0.6d and 6.8±0.1yr, respectively. We suspect that the SD-, or LBV phenomenon is provoked by two types of pulsational modes. The oscillating O-C values for both variables indicate the possible presence of beat cycles. Their time scales are of the order of years to decades. We found, at least for the SD activity of AG Car, further support for a radius change of the star and a more or less horizontal displacement in the theoretical HR-diagram. However, S Dor has a higher luminosity in minimum than in maximum, amounting to 0.5-1mag depending on the range in the visual magnitude (1-2.4mag). Inconsistencies between existing temperature scales during the light variations of AG Car were noticed. We found no cyclic pattern whatsoever in the SD phases of Eta Car. The time scale for such events within the last 20yr lies between 1 and 3yr. There is evidence that over the last 20 yrs the central LBV in the Eta Car system experienced a rising branch of a VLT-SD phase, which appears to furnish, according to the secular colour change, indirect support for an ongoing decrease of the circumstellar dust density.