Additional α(16)Λ(9)-photometry of upper main sequence stars with anomalous abundances confirm our previous results, namely, Ap stars are neatly separated from normal main sequence stars. Furthermore, Ap stars are located on three different zones of the α(16)Λ(9)-plane, according to their abundance anomalies to form three photometric groups, (i) Si stars, (ii) Hg,Mn stars, and (iii) Cr,Eu,Sr stars.

Geneva photometry of the important and relatively well known Ap star 53 Cam (HR 3109, HD 65339) is presented. These 27 data, which cover rather evenly the rotational phase interval, are compared with the photometric data in ten bandpasses published by Musielok et al. (1980). They allow to show that at least four “null-wavelength regions” occur between 3400 and 7800 Å.

The strongest broad absorption feature in the peculiar energy distributions of the Ap stars is that centered at about 5200 Å, thus the Stromgren y band and the Geneva VI band are most affected in stars in which this feature is strong. Fig. 1 shows bandpasses (full width at ½ intensity maximum) of three widely used photometric systems superimposed on two of our scans of Ap stars and two solar abundance line blanketed model atmospheres (Kurucz 1979). It is seen that both the y and VI bands fall entirely within the λ5200 feature. The plot (Fig. 2a) of b-y vs. Tpc (the color temperature of the red end of the Paschen continuum), shows that the b-y colors for most of our sample of Ap stars are displaced to the blue of the b-y, Teff relationship of Relyea and Kurucz (1978). In Fig. 2b, Δ(b−y) = (model b−y) − (observed b−y), for a given temperature is plotted vs. ΔWS2(5200), a spectrophotometric index measuring the equivalent width of the λ5200 feature. There is a strong correlation between Δ(b-y) and ΔWS2(5200), indicating quantitatively the large effect of the λ5200 feature on the y band, previously discussed by Adelman (1979). The deviations in Ap star B2-G values from the normal star B2-G vs. T curve are much less than for b-y, as the Geneva G band is largely outside the λ5200 feature (Fig. 2c). Thus B2-G is a better temperature indicator for Ap stars than is b-y (also see Hauck and North 1982).

Out of 111 CP3 stars 56.7 percent are binaries and further 15.3 percent are probable binaries. The distribution of the periods shows a maximum around 7 days and the deficit of periods < 3 days is remarkable. Systems with periods < 15 days seem to rotate synchronously, while wider binaries do not. Photometric observations confirm these, and, in addition, point to an occurance of slow pulsation.

We describe the procedures developed at the Dominion Astrophysical Observatory to coadd high dispersion coude spectrograms and thus increase the signal-to-noise ratio in photographic data. These techniques can yield low noise spectra over a considerable wavelength range. Preliminary results of the analyses of such coadditions are shown to demonstrate the power of these procedures.

With the announcement in 1978 (Kurtz, 1978) of short periodic photometric variations of HD 101065, the first member of a subgroup of CP2 stars was found. This gained attention due to the possibility of applying the methods of stellar seismology to late A to early F type stars. However, the extremely low photometric amplitude of these stars, which is typically few millimagnitudes in B, and the short periods, ranging from few minutes to about 15 minutes, make these stars difficult to discover and a considerable amount of telescope time is required to accumulate sufficient data for a reliable analysis of the frequency spectrum. As a consequence aliasing imposes serious problems. Synchronous observations from observatories well separated in longitude could overcome this dilemma, and have indeed proven to be sucessful (Kurtz and Seeman, 1983; Kurtz and Balona, 1984; Kurtz, Schneider and Weiss, 1985; Kurtz and Kreidl, 1985).

Spectral line profiles are computed for nonradially pulsating CP2 stars. For a range which currently is thought to be typical for these stars, the influence of six parameters on the line profiles is considered: mode order ℓ and degree m, pulsation velocity amplitude, the angle between the rotation and pulsation axis, the angle between the rotation axis and the line-of-sight, and the phase angle of the rotation. In view of the expected low signal-to-noise ratio of observational data it is investigated to what extent easily measurable, simple quantities can still be useful in discriminating between different modes.

Spectroscopic observations of the B9p star ET and secured at the Bulgarian National Observatory Roshen in the years 1981–1984, consisting of 97 plates, show clearly a radial velocity period of 0.198 d - with a ratio to the photometrical period of exactly 2:1. This behaviour would hint at a close binary system; but there arise difficulties in explaining it by this way because of the extreme short period, so that pulsation must be taken into account. However, besides of the confident period of 0.198 d, there is evidence for shorter periods in the region of 45 min, which may be attributed only to pulsational processes.

The cool Ap star HD 60435 was monitored in a programme of rapid B photometry during 18 nights in January/February 1984, from two stations widely spaced in longitude (the University of Toronto 0.6-m telescope at the Carnegie Southern Observatory (CARSO) on Las Campanas, Chile, and the 0.5-m telescope of the South African Astronomical Observatory ( SAAO)). On six of those nights, contiguous light curves from both sites were obtained.

Fourier analysis of these data confirms the rapid variability first reported by Kurtz (1984) and reveals several additional transient oscillations. HD 60435 exhibits persistent - but modulated - oscillations at a frequency near 1.4 IRHZ (period = 11.9 minutes), and short-lived oscillations at frequencies near 1.1 and 4.2 mHz (periods of 15.2 and 4.0 minutes, respectively). These latter two periods represent the longest and shortest yet observed in the class of rapidly oscillating Ap stars.

We have applied the oblique pulsator model (Kurtz 1982) to the fine-scale splittings detected in the frequency spectra of the 1.4 and 1.1 mHz oscillations. Also, the series of frequencies close to 1.4 mHz which fall into a pattern of roughly equal spacing is compared to such spacings predicted for overtones in pulsating main-sequence A stars (Shibahashi and Saio 1984). Both approaches suggest that HD 60435 is undergoing non-radial pulsations of odd and even degree (probably with ℓ ≲ 3).

The oblique pulsator interpretation of the splittings in the frequency spectrum and the amplitude modulation of the 1.4 mHz oscillations also predict: a rotation period of approximately eight days for this star. Mean photometry of HD 60435, collected by the authors, supports a similar value of 7.7 days for the period.

Analysis of the oscillations is hampered by ambiguities due to daily aliases present in the data, and by the complicated structure and time-dependence of the frequency spectrum. Further observations of HD 60435 are essential if we are to fully understand its rapid variations.

The results of automatic reduction of spectral observations of 53 Cam with high time resolution are presented. The statistical analysis of deviations of individual spectrograms of a star from the averaged one for one night permits one to allocate variable details of spectra and estimate the amplitudes of their variability.

Looking in the literature for short period variations of Ap stars, we found the puzzling case of HD 24975. Used as a comparison star by Weiss (1978), it had been found then to present variations with a peak-to-peak amplitude of 0.01 in U, B and V with a period of about 45mn, but with no clear correlation between the three passbands.

HD 24975 has the same spectral type A2 as 21 Com which seems to present photometric variations with P ⋍ 31mn and Δ V < 0,02m (Percy, 1973, 1975). Both stars are near the blue edge of the δ Scuti instability strip in the HR diagram. The lack of photo metric data, however, prevented the precise location of HD 24975 from being found on the HR diagram.

The present discussion of hot magnetic CP stars is restricted to 17000 K < Teff < 30 000 K, the upper limit being the temperature up to which magnetic fields can be observed, the lower limit defining the boundary between He-rich and He-poor stars.

All of the hot magnetics belong to the class of intermediate helium stars which is defined by nH/nHe, < 3 (number ratio, Hunger 1975). Walborn (1983) lists 23 stars of this class, mostly with nH/nHe, ≈ 1, and of spectral type B2 V. Magnetic fields, so far, have been discovered in 7 of them (Borra et al. 1983). All but one of these are spectrum variables, with periods 0.9<P/day<1.7 (except one with P = 9.5 d), (Pedersen, 1979) and with rotational velocities of the order of v sin i ≈ 150 km/s which are compatible with the (P, v sin i)-relation of stars near the main sequence; i.e. these stars are oblique rotators.

Photometric observations in the ultraviolet region obtained by the ANS satellite enabled us: to confirm with a slight improvement the rotational period of HD 22470 (UV lightcurves of HD 22470 compared with u and magnetic curves are shown in fig. 1); to confirm the variability with very small amplitudes for HD 74196, a member of the open cluster IC 2391 (Fig. 2)

to compare the amplitude-wavelength relations of some other He-weak stars with known periods (HD 35298, HD 142884, HD 144334, HD 175362, and HD 109026). A preliminary rotational period for the star HD 109026 using the 11 ANS observations was found to be about 1.5 days.

This paper contains two parts:

1. 1) A review of the most recent results on the silicon peculiar stars. Five topics will be reviewed: parameters of Bp-Ap Si stars — UV spectrum — spectrophotometry — spectrum variations and flux redistribution -magnetic braking.

2. 2) The horizontal diffusion of the silicon in the presence of a magnetic field and, as a consequence, the evolution of the silicon abundance repartition in the stellar atmosphere. A first observational test is presented that supports the predicted scenario.

The influence of variable blanketing on the photometric variations of hot, silicon Ap star HD 27309 has been estimated. Spectrum variations in the visible are not the principal reason for the observed light variability, but flux redistribution from UV into visible fit well the observational data. An reference is made on the fulfillment of the variable blanketing assumptions at least for silicon Ap stars.

The atmosphere parameters, spectral variability and the magnetic field geometry of the Bp-star θ Aur are evaluated from 30 coude-spectrograms. We adopt Te = 10 800K and lgg = 3.40 (with lower reliability of lg g ) . The equivalent widths of the metalic lines show a relatively great scattering which troubles the period search. The best fit to our data gives the period P = 3.620d, but the variability is clearly seen only for Sill lines. The uncertainties in the star parameters obtained bring to a great uncertainty in the magnetic dipole geometry: i = 86° + 39° and β = 18° + 80°. We adopt the most plausible star parameters to be: R = 3.8R⊙ M = 3.5R⊙, i = 80°, β = 20°, Bp = 1 KGs.

Sixty five TD-1 scans of the magnetic silicon star HD 170000 have been investigated using the method for lightcurves analysis proposed by Hempelmann and Schöneich. The fluxes originating in two spots and in the rest surface of the star have been separated. The conclusion is that the whole stellar surface is strongly peculiar. Spot 1 is a region with the same peculiarity as the rest surface, but strengthened, whereas spot 2 shows additional peculiarities with respect to spot 1 and the rest surface.