Effect of chromospheric activity on the mean colours of late-type stars

We confirm the existence of an " excess" in the mean dereddened ( U - B)(0) 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.