2012A&A...539A..58C

While the search for exoplanets around main sequence stars more massive than the Sun have found relatively few such objects, surveys performed around giant stars have led to the discovery of more than 30 new exoplanets. The interest in studying planet-hosting giant stars resides in the possibility of investigating planet formation around stars more massive than the Sun. Masses of isolated giant stars up to now were only estimated from evolutionary tracks, which led to different results depending on the physics considered. To calibrate the theory, it is therefore important to measure many giant star diameters and masses as independent as possible of physical models. We aim to determine the diameters and effective temperatures of five giant stars, one of which is known to host a planet. We used optical long-baseline interferometry with the aim of testing and constraining the theoretical models of giant stars. Future time-series spectroscopic observations of the same stars will allow the determination of masses by combining the asterosimological analysis and the interferometric diameter. AMBER/VLTI observations with the ATs were executed in low-resolution mode on five giant stars. To measure highly accurate calibrated squared visibilities, a calibrator-star-calibrator observational sequence was performed. We measured the uniform disk and limb-darkened angular diameters of four giant stars. The effective temperatures were also derived by combining the bolometric luminosities and the interferometric diameters. Lower effective temperatures were found compared to spectroscopic measurements. The giant star HD12438 was found to have an unknown companion star at an angular separation of ∼12mas. Radial velocity measurements present in the literature confirm the presence of a companion with a very long orbital period (P∼11.4-years).