Chromospherically active, spotted stars generally become redder as well as fainter when large starspots rotate into view on the stellar disc. However, the RS CVn system UX Ari (a triple-lined system), becomes bluer as it gets fainter. One possible explanation is that hot, bright facular regions accompany the cool, dark photospheric spots of the active component. The bluer flux of the hotter, inactive component does not appear to be sufficient to explain the observed behaviour. We have begun a search for additional chromospherically active stars with a similar relation between colour and brightness, to investigate whether these relations can be explained in the same way. Our results for V711 Tau are presented here, and we conclude that the faculae explanation holds also in this case.
V. Aarum-Ulvås, G. W. Henry "Why do some spotted stars become bluer as they become fainter?" Proc. 13th Cool Stars Workshop, Hamburg, 5–9 July 2004