1997A&A...319..967A

In this paper we show, for the first time, the effect of activity on the (U-B)₀ vs. (B-V)₀ colour-colour diagram for a large sample of late type stars. We clearly see that for single-lined spectroscopic RSCVn systems there exists an ultraviolet excess over that recorded for quiescent stars of the same spectral type. The same effect is seen in main-sequence stars but to a lesser extent. From simple arguments we demonstrate that the cause is unlikely to be unresolved flaring, the combined effect of strong chromospheric emission lines, or X-ray/UV back-heating of the photosphere. White-light faculae are a reasonable candidate for the effect however. In our investigation we have not taken into account the effect that metallicity might have on the colour indices.