M. Breger, W. Ostermann, J. Stich and M. Scheck are continuing the multisite photometric campaigns of selected Delta Scuti stars together with astronomers at other observatories. During January 1990, several weeks of photometry of HR 2724 were obtained by L. Balona at the South African Astronomical Observatory and H.-G. Grothues with the Bochum Telescope at ESO. A preliminary analysis of the data indicates that since 1981 the amplitudes of pulsation have decreased. The multiple frequencies of HR 729, BU Cnc and HD 73819 from last year's campaigns are presently being analyzed. During 1990 May/June, HR 6391 will be observed during a multisite campaign.
The discovery that the amplitudes of many nonradially pulsating Delta Scuti stars are variable on a time scale of years raises the question whether the stars found to be constant during previous variability surveys may in fact show detectable variability a decade or two later. In order to improve the statistics of the incidence of variability inside the instability strip and its dependence on rotational velocity (M. Breger, Astrophys. J. 176, 373, 1972), J. Stich is following up the 1969 variability study of the Praesepe cluster with a new survey.
sperl@astro.ast.univie.ac.at