Photoelectric measurements of the β-index were made for six Ap-stars (56 Ari, 41 Tau,φDra, HD 188041, HD 215441, HD 221568) and one He-rich star (HD 184927). For all these stars the minimum of β occurs at a phase of maximum light at a wavelength longward of the null-wavelength region. Such coincidence can be explained by the blanketing mechanism proposed by Kodaira (1973) for the explanation of the Hγ line variations in HD 221568. According to Kurucz (1979), for stars hotter than 9000K an

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... se of metal abundance in the atmosphere causes an increase of the visible flux and an simultanous decrease of equivalent widths of Balmer lines. The same changes of the visible flux and equivalent widths of Balmer lines can be obtained using a model atmosphere with a suitable higher temperature. Using the Kurucz (1979) model atmospheres, temperature differences were calculated, which are necessary to obtain the observed decrease of the (β-index and the increase of the flux in a given photometric band.