  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 a large number of giant star
diameters and masses as much as possible independent of physical models. We aim
in the determination of diameters and effective temperatures of 5 giant stars,
one of which is known to host a planet. AMBER/VLTI observations with the ATs
were executed in low resolution mode on 5 giant stars. In order to measure high
accurate calibrated squared visibilities, a calibrator-star-calibrator
observational sequence was performed. We measured the uniform disk and
limb-darkened angular diameters of 4 giant stars. The effective temperatures
were also derived by combining the bolometric luminosities and the
interferometric diameters. Lower effective temperatures were found when
compared to spectroscopic measurements. The giant star HD12438 was found to
have an unknown companion star at an angular separation of ~ 12 mas. Radial
velocity measurements present in the literature confirm the presence of a
companion with a very long orbital period (P ~ 11.4 years).}
