Flaring loop structures at VLBI scale in UX Arietis

We report the results of three VLBI observations at 5 GHz of the active bin ary system UX Ari. Observations have been made during five consecutive days with the aim of studying the variations of the source structure during the decay phase of a flare. We find that during the observations UX Ari was undergoing a low-energy fla re (peak flux similar to 60 mJy) with a decay time of a few days. The maps show a clear variation of the source structure with time. The visibility fu nction and the maps can be reproduced using two gaussian components of angu lar dimensions similar to 2 x 1 mas, whose relative position changes. Since the observations have been made at different orbital phases, we interpret the observed variations of the source structure as due to two distinct flar ing loops that are seen in different positions as the star rotates.