THE PURE DIPOLE PULSATION MODE OF THE RAPIDLY OSCILLATING AP STAR-ALPHA-CIR - DETERMINATION OF THE ROTATION PERIOD, AND EVIDENCE FOR FREQUENCY VARIABILITY

New, multisite observations of the rapidly oscillating Ap star alpha Cir (HR5463, HD 128898) show that its principal pulsation mode is a pure oblique dipole mode with a frequency of 2442 mu Hz. A rotationally split triplet shows the rotation period to be P-rot = 4.463 +/- 0.010 d. Reanalysis of Johnson B data spanning the years 1981 to 1993 refines the rotation period to P-rot= 4.4790 +/- 0.0001 d. Four low-amplitude secondary frequencies are present, with separations that are multiples of 25 mu Hz. From this we suggest that Delta v(0) = 50 mu Hz. The asteroseismological luminosity inferred from this agrees with the luminosity determined independently from the parallax. The principal frequency in alpha Cir is variable. The available data suggest that this variability is cyclic on a time-scale of hundreds of days. Cyclic frequency variability appears to be the norm in rapidly oscillating Ap stars; since binary motion can be ruled out in some cases, and shown to be extremely unlikely in other cases, this frequency variability is intrinsic to these stars.