  Aims: We aim at detecting and characterizing the main-sequence companion of
the Cepheid AX Cir ($P_\mathrm{orb} \sim $ 18 yrs). The long-term objective is
to estimate the mass of both components and the distance to the system.
Methods: We used the PIONIER combiner at the VLT Interferometer to obtain the
first interferometric measurements of the short-period Cepheid AX Cir and its
orbiting component. Results: The companion is resolved by PIONIER at a
projected separation $\rho = 29.2 \pm 0.2$ mas and projection angle $PA = 167.6
\pm 0.3^{\circ}$. We measured $H$-band flux ratios between the companion and
the Cepheid of $0.90 \pm 0.10$ % and $0.75 \pm 0.17$ %, respectively at a
pulsation phase for the Cepheid $\phi = 0.24$ and 0.48. The lower contrast at
$\phi = 0.48$ is due to increased brightness of the Cepheid compared to the
$\phi = 0.24$. This gives an average apparent magnitude $m\mathrm{_H (comp)} =
9.06 \pm 0.24$ mag. The limb-darkened angular diameter of the Cepheid at the
two pulsation phases was measured to be $\theta_\mathrm{LD} = 0.839 \pm 0.023$
mas and $\theta_\mathrm{LD} = 0.742 \pm 0.020$ mas, respectively at $\phi =
0.24$ and 0.48. A lower limit on the total mass of the system was also derived
based on our measured separation, we found $M_\mathrm{T} \geq 9.7 \pm 0.6
M_\odot$.
