  Physical processes working in the stellar interiors as well as the evolution
of stars depend on some fundamental stellar properties, such as mass, radius,
luminosity, and chemical abundances. A classical way to test stellar interior
models is to compare the predicted and observed location of a star on
theoretical evolutionary tracks in a H-R diagram. This requires the best
possible determinations of stellar mass, radius, luminosity and abundances. To
derive its fundamental parameters, we observed the well-known rapidly
oscillating Ap star, $\gamma$ Equ, using the visible spectro-interferometer
VEGA installed on the optical CHARA array. We computed the calibrated squared
visibility and derived the limb-darkened diameter. We used the whole energy
flux distribution, the parallax and this angular diameter to determine the
luminosity and the effective temperature of the star. We obtained a
limb-darkened angular diameter of 0.564~$\pm$~0.017~mas and deduced a radius of
$R$~=~2.20~$\pm$~0.12~${\rm R_{\odot}}$. Without considering the multiple
nature of the system, we derived a bolometric flux of $(3.12\pm 0.21)\times
10^{-7}$ erg~cm$^{-2}$~s$^{-1}$ and an effective temperature of
7364~$\pm$~235~K, which is below the effective temperature that has been
previously determined. Under the same conditions we found a luminosity of
$L$~=~12.8~$\pm$~1.4~${\rm L_{\odot}}$. When the contribution of the closest
companion to the bolometric flux is considered, we found that the effective
temperature and luminosity of the primary star can be, respectively, up to
$\sim$~100~K and up to $\sim$~0.8~L$_\odot$ smaller than the values mentioned
above.These new values of the radius and effective temperature should bring
further constraints on the asteroseismic modelling of the star.

Solar and Stellar Astrophysics