ON THE ORIGIN OF OPTICAL VARIABILITY OF WW VULPECULAE

The UBV observations of WW Vul performed between 1983 and 1993 are pre sented, and all of the published UBV observations of the star (more th an 1000 measurements) are analyzed, It is shown that the popular dust hypothesis does not explain all features of the light and color curves of the star at minima and can be adopted only for a magnitude brighte r than V = 11.(m)7. All of the minima with V greater than or equal to 12(m) observed over the last 26 years can be divided into three types: (1) ''red'' minima, when there was absolutely no blue (more precisely , ultraviolet) excess; these minima, along with the light variability in the range 10.(m) 3-11.(m)7, can be produced, in principle, by varia tions in selective absorption; (2) ''violet'' minima, when there was a n ultraviolet excess either in the form of ultraviolet flares with an amplitude Delta(U-B) up to 0.(m)6 or as an additional component persis ted for ten days or more; the B-V color indices at these minima were a pproximately the same as those at the red minima, but U-B were conside rably smaller; and (3) ''blue'' minima, when the color indices were co rrelated in such a way that U-B decreased proportionally to B-V and vi ce versa, but, at the same time, there was no correlation with brightn ess. These features are difficult, if at all possible, to account for in the model of the dust hypothesis. The model of a spotted photospher e, in which convection on a part of the stellar surface is suppressed by the magnetic field, is more viable. This model qualitatively explai ns almost all features of the photometric, colorimetric, and, possibly , polarimetric behavior of the star. As in the dust model, the existen ce of a second variability mechanism-emission of hot gas-is needed.