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 C ir (HR5463, HD 128898) show that its principal pulsation mode is a pur e oblique dipole mode with a frequency of 2442 mu Hz. A rotationally s plit 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 multiple s of 25 mu Hz. From this we suggest that Delta v(0) = 50 mu Hz. The as teroseismological luminosity inferred from this agrees with the lumino sity determined independently from the parallax. The principal frequen cy in alpha Cir is variable. The available data suggest that this vari ability is cyclic on a time-scale of hundreds of days. Cyclic frequenc y 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 ex tremely unlikely in other cases, this frequency variability is intrins ic to these stars.