1997A&AS..124..517V

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 ∼0-2) 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 ∼100d; 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.