  We have used the Sydney University Stellar Interferometer (SUSI) to measure
the angular diameter of alpha Cir. This is the first detailed interferometric
study of a rapidly oscillating A (roAp) star, alpha Cir being the brightest
member of its class. We used the new and more accurate Hipparcos parallax to
determine the radius to be 1.967+-0.066 Rs. We have constrained the bolometric
flux from calibrated spectra to determine an effective temperature of 7420+-170
K. This is the first direct determination of the temperature of an roAp star.
Our temperature is at the low end of previous estimates, which span over 1000 K
and were based on either photometric indices or spectroscopic methods. In
addition, we have analysed two high-quality spectra of alpha Cir, obtained at
different rotational phases and we find evidence for the presence of spots. In
both spectra we find nearly solar abundances of C, O, Si, Ca and Fe, high
abundance of Cr and Mn, while Co, Y, Nd and Eu are overabundant by about 1 dex.
The results reported here provide important observational constraints for
future studies of the atmospheric structure and pulsation of alpha Cir.
