2003A&A...404..631A

We confirm the existence of an ``excess'' in the mean dereddened (U-B)<sub>0</sub> colour of active single-lined spectroscopic giants over quiescent stars of the same spectral type and find evidence for this ``excess'' in (B-V)₀, although at a lower scale, when plotted against (R-I)₀. This result suggests that the use of (B-V)₀ calibrations to determine fundamental parameters or spectral types for late-type active stars should be taken with caution. Since magnetic activity is definitely involved, these and similar calibrations using (U-B)₀ and/or (B-V)₀ should not be used for the study of late-type young clusters' members or late-type pre-main sequence stars without some consideration. Instead, near-infrared colours like (R-I)₀ should be better temperature indicators for these stars.