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

¹ Leiden Observatory, Postbus 9513, 2300 RA Leiden, The Netherlands
² Armagh Observatory, College Hill, Armagh BT61 9DG, Northern Ireland
³ University of Brussels (VUB), Pleinlaan 2, 1050 Brussels, Belgium

Send offprint request to: A.M. van Genderen

Received: 8 July 1996
Accepted: 17 December 1996

Abstract

We investigated the photometric histories of the LBVs HR Car in the Galaxy, R 127, R 110 and R 71 in the LMC, and R 40 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 25 y. 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.