NEW PERCEPTIONS ON THE S-DORADUS PHENOMENON AND THE MICRO VARIATIONS OF 5 LUMINOUS BLUE VARIABLES (LBV)

We investigated the photometric histories of the LBVs HR Car in the Ga laxy, 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 be tween 1.4 and 25 y. During the SD cycles two different types of micro- variations (amplitudes similar to 0(m).2) are present: one near the mi nima (time-scale from 2 to 6 weeks; colours generally blue in the maxi ma and red in the minima) and the other one near the maxima (time-scal e similar to 100 d; colours generally red in the maxima and blue in th e minima). Halfway the ascending and descending branches of the SD cyc les the stars switch rather abruptly from one type to the other. We ar gue that the two types of SD phases are probably caused by different i nstability mechanisms.