Sakurai's object (V4334 Sgr) in 1998: The R CrB phase has set in

Photometric BVRI observations of V4334 Sgr during 43 nights in 1998 are pre sented. They show that the star has entered the R CrB phase. Three deep min ima with amplitudes between 1(m) and 7(m) in V were observed in 1998. The c hange in B-V, V-R, and V-l reflected mainly the reddening in the circumstel lar envelope and, possibly, further cooling of the star. The previously obs erved light variations are found to be described by a systematic increase o f the only pulsation period from 9 to 40 days in late 1997 and, possibly, t o 60 days in late 1998. By the C-2 and CN absorption bands and the Na I lin es, in 1998 the star is classified as C-J4 C(2)5 j5 in Keenan's revised sys tem. The continuum energy distribution in the wavelength range 4500-7500 An gstrom in late April 1998 is fitted by the spectral energy distribution of a G5 Ib supergiant. The extinction law in the circumstellar dust envelope o f V4334 Sgr is shown to essentially match the interstellar extinction law a t optical wavelengths. The luminosity of the star in 1996-1997 was constant if its effective temperature corresponded to the scale of normal supergian ts and if the interstellar reddening is assumed to be E(B-V) = 0.8.