  We present a detailed study of the pulsation of alpha Circini, the brightest
of the rapidly oscillating Ap stars. We have obtained 84 days of high-precision
photometry from four runs with the star tracker on the WIRE satellite.
Simultaneously, we collected ground-based Johnson B observations on 16 nights
at the South African Astronomical Observatory. In addition to the dominant
oscillation mode at 2442 microHz, we detect two new modes that lie
symmetrically around the principal mode to form a triplet. The average
separation between these modes is 30.173+-0.004 microHz and they are nearly
equidistant with the separations differing by only 3.9 nHz. We compare the
observed frequencies with theoretical pulsation models based on constraints
from the recently determined interferometric radius and effective temperature,
and the recently updated Hipparcos parallax. We show that the theoretical large
separations for models of alpha Cir with global parameters within the 1-sigma
observational uncertainties vary between 59 and 65 microHz. This is consistent
with the large separation being twice the observed value, indicating that the
three main modes are of alternating even and odd degrees. The frequency
differences in the triplet are significantly smaller than those predicted from
our models, for all possible combinations of mode degrees, and may indicate
that the effects of magnetic perturbations need to be taken into account. The
WIRE light curves are modulated by a double wave with a period of 4.479 days,
and a peak-to-peak amplitude of 4 mmag. This variation is due to the rotation
of the star and is a new discovery, made possible by the high precision of the
WIRE photometry. The rotational modulation confirms an earlier indirect
determination of the rotation period.
