I. Stable stars

Context. Future astrometric missions such as SIM PlanetQuest need very stable reference stars. K giants have large luminosities, which place them at large distances and thus the jitter of their photocenters by companions is relatively small. Therefore K giants would be best suited as references. To confirm this observationally a radial velocity survey is performed to quantify the level of intrinsic variability in K giants. Aims. From this radial velocity survey we present 34 K giants with an observed standard deviation of the radial velocity of less than 20 m/s. These stars are considered “stable” and can be used as radial velocity standards. Methods. The radial velocity survey contains 179 K giants. All K giants have a declination between −30 and +65 and visual magnitude of 3 − 6 mag. The Coudé Auxiliary Telescope (CAT) at UCO/Lick Observatory is used to obtain radial velocities with an accuracy of 5 − 8 m/s. The number of epochs for the 34 stable stars ranges from 11 to 28 with a total timespan of the observations between 1800 and a little over 2200 days. Results. The observational results of the 34 “stable” stars are shown together with a discussion about their position in the MV vs. B−V diagram and some conclusions concerning the radial velocity variability of K giants. These results are in agreement with the theoretical predictions. K giants in a certain range of the MV vs. B − V diagram are suitable reference stars.