THE RISE AND FALL OF V4334 SAGITTARII (SAKURAIÏS OBJECT)

CCD UBV Ri photometry of the Ðnal helium Ñash object V4334 Sgr (SakuraiÏs object) carried out during 1997È1999 is presented, and the light curve from its prediscovery rise to the dust obscuration phase is constructed. The optical light curve can be divided into four sections, the rise to maximum, the maximum, the dust onset, and the massive dust shell phase. The color indices show a general increase with time, Ðrst because of the photospheric expansion and cooling and later because of the dust-forming events. The energy distributions for the years 1996È1999 show that an increasing part of the energy is radiated at infrared wavelengths. In 1996 the infrared excess is likely caused by free-free radiation in the stellar wind. Starting from 1997 or 1998 at the latest, carbon dust grains are responsible for the more and more dramatic decrease of optical radiation and the growing infrared excess. Its photometric behav-ior in 1998È1999 mimics the red declines ÏÏ of R CrB variables; the amplitude, however, is more extreme than any fading ever observed in an R CrB star. Evidence is given that a complete dust shell has formed around V4334 Sgr. It therefore shows similarities with dust-forming classical novae, although it is evolv-ing D20 times more slowly. Its luminosity increased by a factor 4 between 1996 and 1998. A comparison of timescales of the Ðnal helium Ñash objects FG Sge, V605 Aql, and V4334 Sgr shows that the observed photometric and spectroscopic features are similar, while V4334 Sgr is the most rapidly evolving object to date