Long-term photometry of Be stars. I. Fading events and variations on time scales of years.

Stroemgren differential uvby photometry of 7 Be stars was analyzed. The data were taken during 1983-1991 by the Long-Term Photometry of Variables team at La Silla. Two low v.sin(i) stars (27 CMa and 28 CMa) showed fading events-that is, sudden fading of a few weeks duration-that were preceded by quasi-periodic oscillations (QPO) on time scales T~10-20 days and amplitudes up to 0.2mag. In both cases the color variations indicated occurrence of dust ejections. QPO were observed also in 48 Lib and in ɛ Cap, but these were not related to any fading event. Long-term photometric variations up to 0.6mag amplitude were seen for all stars. In most cases, these variations were larger in the u band, the stars being bluest when brightest. The Be stars seems to follow a fixed track in the c1-(b-y) color plane. The b-y color is not very sensitive to changes in brightness, whereas the c1 color is strongly correlated with the u magnitude. Moreover, the largest c1 changes, compared with u changes, occurred in high v sin i Be stars with shell characteristics. Additional variations on time scales of days were always present; these were larger in the u band. HR 3237 shows peculiar photometric variations, being the exception to the behaviour above described. The amplitudes of short-term and long-term variations of 13 Be stars are correlated; the tightest correlation was found in the y band. We discuss long-term photometric variations and quasi-periodic oscillations as due to enhanced mass loss episodes preceded by global one-armed thin-disk oscillations, which could be excited by some type of photospheric activity.