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A&A Supplement Ser., Vol. 124, September 1997, 517-531

Received July 8; accepted December 17, 1996

New perceptions on the S Doradus phenomenon and the micro variations of five Luminous Blue Variables (LBVs)

A.M. van Genderentex2html_wrap1570 - M. de Groottex2html_wrap1572 - C. Sterkentex2html_wrap1574gif

Send offprint request: A.M. van Genderen

tex2html_wrap1576  Leiden Observatory, Postbus 9513, 2300 RA Leiden, The Netherlands
tex2html_wrap1578  Armagh Observatory, College Hill, Armagh BT61 9DG, Northern Ireland
tex2html_wrap1580  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 tex2html_wrap_inline1556) 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 tex2html_wrap_inline1558; 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

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