Periods, period changes and the nature of the microvariations of Luminous Blue Variables

We present period determinations of the microvariability of the six luminous blue variables AG Car, HR Car, 164 G Sco, S Dor, R 127, and R 71. In total, we were able to determine 22 periods in these stars, ranging from 18 days up to 195 days. All stars have period changes by up to a factor 4 within time scales of a few hundred days. For all stars the amplitude of the pulsations in V increases with increasing periods. The slope of the correlation between the amplitude and the period decreases with increasing luminosity. The values of the pulsation constant Q were determined. HR Car, 164 G Sco, R 71 and R 127 have Q-values in the range of 0.07 to 0.18 days. This is about a factor two larger than those of most other B-type supergiants, possibly because the LBVs have a higher L/M ratio as they have lost more mass. The most common value for the pulsational constant of LBVs is Q = 0.07 +/- 0.01 days, but Q can increase temporarily by as much as a factor four. This is not related to a particular phase in the light curve. The long periods might be due to a beat of two frequencies. For the two stars R71 and R 127, which showed significant changes in M-V, and hence in radius during the course of the observations, the pulsational period increased with increasing radius. The Q-values of R 71 and R 127 increase when the stars get brighter and their radii increase. This is probably due the changes in the density structure of the stars as their outer envelope expands. We compare the observed variations with those predicted for strange modes by Kiriakidis et al. (1993). The periods of the observed microvariations are orders of magnitudes longer than predicted for strange modes. A comparison with the variations of slowly pulsating B-stars (SPBs) suggests that the microvarions of LBVs are due to g-mode pulsations. A first attempt for mode identification, based on a simple linear pulsation model by means of the multicolour Stromgren data, shows that none of the variations can be explained by means of a radial pulsation. The amplitude-wavelength relations suggest g-modes of low l.