AN UNUSUAL BRIGHTENING OF THE ECLIPSING BINARY STAR AKO-9 IN THE GLOBULAR-CLUSTER 47-TUCANAE OBSERVED WITH THE HUBBLE-SPACE-TELESCOPE

Fifteen sequential images of the core of the globular cluster 47 Tucan ae taken in ultraviolet light with the Hubble Space Telescope have rev ealed one star that increased in brightness by more than 2 mag in less than an hour. By the end of our observations, this star was the brigh test object in the core of the cluster in our bandpass. Auriere et al. first cataloged this star as a blue object (AKO 9), considering it as a potential visible counterpart of the then still single X-ray source . Edmonds et al. found it to be an eclipsing binary, located in the co lor-magnitude diagram in the vicinity of the main-sequence turnoff. Po ssible causes for such a brightening are (1) a very large flare on a m agnetically active star (RS CVn), (2) an increase in the brightness du e to an accretion disk instability in a cataclysmic variable or (3) in a soft X-ray transient, or (4) a nova. There are arguments against ev ery one of these possibilities, and more observations will be needed t o understand this system.