RADIAL PULSATION AND THE ROTATION PERIOD OF THE RAPIDLY OSCILLATING AP STAR ALPHA-CIRCINI (HR-5463, HD-128898)

A frequency analysis of 211 h of high-speed photometric observations o btained in 1981, 1983 and 1992 shows that alpha Cir pulsates in two mo des with frequencies nu1 = 2442.023 +/- 0.016 muHz and nu2 = 2439.427 +/- 0.016 muHz, separated by nu1 - nu2 = 2.60 +/- 0.02 muHz. This sepa ration suggests that nu1 is a mode of overtone n almost-equal-to 35-40 and degree l = 0, while nu2 has overtone and degree of n - 1 and l = 2. The principal frequency, nu1, however, is phase-modulated. The modu lation period-may be 524.2 d, in which case alpha Cir is a binary star with a reasonable probability that the secondary is of substellar mas s. The modulation period may be 0.995 350 d, which is then the rotatio n period, in which case it can be shown that the principal frequency c annot be represented purely by a radial spherical harmonic pulsation m ode. A decomposition of a derived frequency triplet indicates that, fo r most values of the rotational inclination i and the magnetic obliqui ty beta, the mode is mostly a radial mode with a smaller dipole distor tion. We suggest that the rotational inclination i less-than-or-equal- to 12-degrees and the magnetic obliquity is large, which means that th e magnetic field is seen always from an unfavourable aspect. The sugge sted rotation period, P(rot) = 0.995 350 +/- 0.000 005 d, is so close to one sidereal day that it must be confirmed with multisite observati ons. If it is not confirmed, then the binary model becomes more attrac tive.