A Unique Dust Formation Episode in the SC-Type Star UY Cen

We report the first detection of new dust formation in an SC star. The prototype of the SC stars, UY Cen, underwent a decline of 2 magnitudes in the V-band. The SC stars show pulsational variations and have 60 micron excesses indicating past dust formation. It has been suggested that as a star evolves from oxygen rich to carbon rich, there is a short period of time when C/O$~1 that the star appears spectroscopically as an SC star and ceases to produce dust. The SC star, BH Cru, has shown large spectroscopic and pulsation period variations in only 30 years, indicating rapid evolution but it has shown no sign of new dust formation. UY Cen has not shown any pulsation or spectroscopic variations accompanying the onset of its dust formation. In addition, UY Cen did not show emission in the resonance lines of Na I, K I or Rb I when it was at its faintest, although these lines were a feature of the carbon stars R Lep and V Hya during similar faint phases.