Bluer colour of UX Arietis at fainter visual magnitudes

Abstract

Aims.The RS CVn star UX Ari appears bluer when it is fainter, which is unusual for a spotted star; we aim to resolve this unusual behaviour of the object. Also the data available in the literature indicate that the mean V magnitude of UX Ari varies by about 25 years; we want to confirm whether this is indeed true. Methods.The most important parameter in understanding the colour variation is the relative brightnesses of the component stars of UX Ari. We use all spectroscopic information available in the literature to obtain a reasonable value for this parameter. We also present new BV photometry of UX Ari obtained on 58 nights during 2001-07. Results.The V magnitudes of UX Ari we obtained are significantly fainter than those reported in the literature, and they are not consistent with a 25-year period for the mean light level. The increased fractional contribution by the hotter G-type component to the total light in the blue spectral region as the active star becomes faint, seems to be the real cause for the bluer colour of UX Ari at fainter visual magnitudes.