  The well-known light minima of stars with the R Coronae Borealis variability
are caused by the formation of an additional circumstellar dust shell, the
screening shell, inside the permanent shell. Under the assumption of uniform
distribution of matter in the circumstellar environment we estimated the
optical thickness of the permanent gas-and-dust shell at 0.2-0.7, and its
geometrical thickness is no less than 0.4 of its own radius. The wavelength
dependence of extinction is close to neutral. From spectral observations of R
CrB itself in the 1985 minimum we traced the transformation of the stellar
linear and molecular absorption spectrum to the emission spectrum and
established that the fast variation of the U-B colour index by -0.6 in the
light decline was caused purely by a change of the spectrum type. The spectrum
transformation causes an increase of star brightness in the U, B, and V bands
by about 1.4, 0.75, and 0.75 mags, correspondingly. It is suggested that a
high-velocity (>200 km/s) matter stream through the circumstellar environment
is the cause of the excitation of the emissions observed during light minima
when the photospheric flux is weakening.
