Abstract

Aims. We aim at detecting and characterizing the main-sequence companion of the Cepheid AX Cir (P-orb similar to 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 +/- 0.2 mas and projection angle PA = 167.6 +/- 0.3 degrees. We measured H-band flux ratios between the companion and the Cepheid of 0.90 +/- 0.10% and 0.75 +/- 0.17%, at pulsation phases for the Cepheid of phi = 0.24 and 0.48, respectively. The lower contrast at phi= 0.48 is due to the increased brightness of the Cepheid compared to phi = 0.24. This gives an average apparent magnitude m(H)(comp) = 9.06 +/- 0.24 mag. The limb-darkened angular diameter of the Cepheid at the two pulsation phases was measured to be theta(LD) = 0.839 +/- 0.023 mas and theta(LD) = 0.742 +/- 0.020 mas, at phi = 0.24 and 0.48, respectively. A lower limit on the total mass of the system was also derived based on our measured separation, and we found M-T >= 9.7 +/- 0.6 M-circle dot.