A&A Supplement Ser., Vol. 124, September 1997, 517-531
Received July 8; accepted December 17, 1996
A.M. van Genderen
- M. de Groot
- C. Sterken
Send offprint request: A.M. van Genderen
Leiden Observatory, Postbus 9513, 2300 RA Leiden, The
Netherlands
Armagh Observatory, College Hill, Armagh BT61 9DG, Northern Ireland
University of Brussels (VUB), Pleinlaan 2, 1050 Brussels, Belgium
We investigated the photometric histories of the LBVs HR Car in the
Galaxy, R127, R110 and R71 in the LMC, and R40 in the SMC by
collecting all available photometry. All these objects have been reasonably
well observed over the last few decades and a number of "S Dor (SD)
phases'' (episodes of enhanced light) is well documented. Time-scales
lie between 1.4 and 25y. During the SD cycles two different
types of micro-variations (amplitudes 
) are present: one
near the minima (time-scale from 2 to 6 weeks; colours generally blue
in the maxima and red in the minima) and the other one near the maxima
(time-scale 
; colours generally red in the maxima and blue
in the minima). Halfway the ascending and descending branches of the SD
cycles the stars switch rather abruptly from one type to the other. We
argue that the two types of SD phases are probably caused by different
instability mechanisms.
keywords: technique: photometric -- stars: individual HR Car = HD 90177, R127 = HDE 269858f, R40 = HD 6884, R71 = HDE 269006, R110 = HDE 269662 -- stars: variables -- stars: supergiants -- stars: oscillations