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Optical-infrared interferometry can provide direct geometrical measurementsof the radii of Cepheids and/or reveal unknown binary companions of thesestars. Such information is of great importance for a proper calibration ofPeriod-Luminosity relations and for determining binary fraction among Cepheids.We observed the Cepheid X Sgr with VLTI/AMBER in order to confirm or disprovethe presence of the hypothesized binary companion and to directly measure themean stellar radius, possibly detecting its variation along the pulsationcycle. From AMBER observations in MR mode we performed a binary model fittingon the closure phase and a limb-darkened model fitting on the visibility. Ouranalysis indicates the presence of a point-like companion at a separation of10.7 mas and 5.6 magK fainter than the primary, whose flux and position aresharply constrained by the data. The radius pulsation is not detected, whereasthe average limb-darkened diameter results to be 1.48+/-0.08 mas, correspondingto 53+/-3 R_sun at a distance of 333.3 pc.

On the binarity of the classical Cepheid X Sgr from interferometric observations