Top: histogram showing the observed velocity dispersions (σ_RV) for the MS stars (pink) and giants and turn-off stars (blue) of our sample. Binary candidates and stars with high RV trend have been excluded. The black line represents the Gaussian fit centered at ~21  m s^-1 of width σ ~ 7  m s^-1. Bottom: histogram showing the ratio of the RV dispersion (σ_RV) and the average measurements error (σ_obs). The symbols are the same as in the top panel.