Radial-velocity standard deviation (σ(RV), or σ(O−C) in case of orbital SBs) vs. luminosity for sample S1. Symbols are as follows: filled squares show stars with small-amplitude variations, large open circles signify SBs, crosses represent stars showing jitter, open squares show stars with no special features. We note that the jitter stars and those with small-amplitude RV variations are among the most luminous ones, as expected.