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 \mu Hz , we detect two new modes that lie symmetrically around the principal mode to form a triplet .
The average separation between these modes is \Delta f = 30.173 \pm 0.004 \mu Hz 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 \mu Hz .
This is consistent with the large separation being twice the observed value of \Delta f , 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 d , 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 .
The main pulsation mode at 2442 \mu Hz has two sidelobes split by exactly the rotation frequency .
From the amplitude ratio of the sidelobes to the central peak we show that the principal mode is consistent with an oblique axisymmetric dipole mode ( l = 1 ,m = 0 ) , or with a magnetically distorted mode of higher degree with a dominant dipolar component .