2014A&A...561L...3G

We aim at detecting and characterizing the main-sequence companion of the Cepheid AX Cir (Porb ∼18yrs). The long-term objective is to estimate the mass of both components and the distance to the system. 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. The companion is resolved by PIONIER at a projected separation ρ= 29.2 ±0.2mas and projection angle PA = 167.6 ±0.3°. 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 φ= 0.24 and 0.48, respectively. The lower contrast at φ= 0.48 is due to the increased brightness of the Cepheid compared to φ= 0.24. This gives an average apparent magnitude m_H (comp) = 9.06 ±0.24mag. The limb-darkened angular diameter of the Cepheid at the two pulsation phases was measured to be θLD = 0.839 ±0.023mas and θLD = 0.742 ±0.020mas, at φ= 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 MT ≥9.7 ±0.6 M_☉.