Photometric and colorimetric changes of eruptive stars at the stage of circumstellar dust envelope formation. PU Vul at the time of the deep minimum in 1980

The model of circumstellar dust envelope with the size of particles changing in the time gives an excellent explanation of photometric and colorimetric behaviour of PU Vul at the stage of circumstellar dust envelope formation: 1) the time-dependent light minima in various spectral bands; 2) the reddening of the star at the decline of light and the bluing at the bottom of the light minimum; 3) the rapid variations of light and colors at the minimum level of star's brightness; 4) the shape of the magnitude - color diagram. The conclusion was made from the comparison of observations data and calculated curves that the deep minimum of PU Vul in 1980 was due to the formation near the hot component of the system, of a circumstellar envelope of silicate particles grwoing from molecular size to the size of interstellar grains. The mean rate of grwoth was approximately 0.001 μm/day