  We present new constraints on the rapidly oscillating Ap star alpha Cir,
derived from a combination of interferometric and photometric data obtained
with the Sydney University Stellar Interferometer (SUSI) and the WIRE
satellite. The highlights of our study are:
  1. The first determination of the angular diameter of an roAp star.
  2. A nearly model-independent determination of the effective temperature of
alpha Cir, which is found to be lower than previously estimated values.
  3. Detection of two new oscillation frequencies allowing a determination of
the large separation of alpha Cir.
  Based on this new information, we have computed non-magnetic and magnetic
models for alpha Cir. We show that the value of the observed large separation
found from the new data agrees well with that derived from theoretical models.
Moreover, we also show how the magnetic field may explain some of the anomalies
seen in the oscillation spectrum and how these in turn provide constraints on
the magnitude and topology of the magnetic field.
