2013A&A...549A..64L

Optical-infrared interferometry can provide direct geometrical measurements of the radii of Cepheids and/or reveal any unknown binary companions of these stars. Such information is very important for properly calibrating the period-luminosity relations and determining binary fraction among Cepheids. We observed the Cepheid X Sgr with VLTI/AMBER in order to confirm or disprove the presence of the hypothesized binary companion and to directly measure the mean stellar radius, possibly detecting its variation along the pulsation cycle. From AMBER observations in MR mode, we performed a binary model fitting on the closure phase and a limb-darkened model fitting on the visibility. Our analysis indicates that there is a point-like companion at a separation of 10.7mas, which is 5.6mag_K fainter than the primary and whose flux and position are sharply constrained by the data. The radius pulsation is not detected, whereas the average limb-darkened diameter results in 1.48±0.08mas, corresponding to 53±3R_☉ at a distance of 333.3 pc.