On the basis of lUE data, the specific absorption features in the spectra of Ap-Si Stars are displayed in the 1250–1850 Å range and identified when possible. The contribution of the Si II multiplets is calculated in LTE for a typical overabundance × 102 of silicon. It accounts for only a small part of the observed absorption. Common unidentified structures, smaller than the 1400 Å depression are pointed out.

Progress in the spectroscopic study of CP stars and related sharp-lined normal stars from the IUE is briefly reviewed as a preamble to a discussion of the potential for research with the Hubble Space Telescope. The substantial gains in spectral resolution, signal-to-noise ratio and photometric accuracy that will be realized with the High Resolution Spectrograph on the HST will dramatically increase our ability to disentangle the complex ultraviolet spectra of these stars and to carry out accurate quantitative analyses.

The formulation and solution of the inverse problem to determine local Stokes parameters on the surface of a star, in particular magnetic chemically peculiar star, and hence to determine surface abundance distribution and magnetic field geometry is reviewed.

The applications of the regularizing algorithms to the solving of inverse problems in astrophysics are presented.

Very accurate spectroscopic line profiles have been obtained at various phases during the 3d.618 period of the magnetic variable Ap-star Θ Aur. These profiles were observed using a Reticon detector at the coudé focus of the 3.5 metre Canada-France-Hawaii telescope. The mapping of Si, Cr and Fe over the surface of the star was done by solving the Inverse Problem. Complex spotty structure has been revealed with the number of Fe spots found being as great as six. The distribution of Cr is found to be similar to Fe but with less detailed structure. Si is distributed quite differently from Fe and Cr. Discussion of the relationship of the magnetic field maximum phase and the light curve along with the maps of the distribution of elements suggests that the principal spots of Fe and Cr are in phase with the light variability but they are 90° out of phase with Si and the magnetic field variation. We are surprised by the Si variability seeming to be 90° out of phase with the light variability.

53 Cam is a well-known magnetic, spectroscopic and photometric variable star of Ap-type. The line intensities of 53 Cam show a variation, which has a period of 8h. The oblique-rotator model, developed mathematically by Deutsch /1970/, has been applied for the spectral and magnetic variations. In this paper, using Deutsch’s method, the distribution of CrII, EuII, TiII, SrII on the stellar surface is analysed and the relation between the abundance of elements and the magnetic structure is studied. The Laplace and Fourier coefficients for each elements have been computed and also the distribution of the elements at the stellar surface has been drawn by a Sinclair ZX Spectrum personal computer.

The influence of the non homogeneous distribution of the abundance of elements on the magnetic field measurements in chemically peculiar stars is discussed.

This review is concerned with the following topics: 1. The occurrence of chemical peculiar stars in binaries; 2. Determination of masses of CP stars; 3. Determination of radii of CP stars; 4. CP stars in clusters. The paper emphasizes the literature from the 23rd Liege Colloquium in 1981 on, and does not refer systematically to earlier papers.

The observational results for 108 stars, members of 10 galactic groups at the age range of lg t = 6,4 - 8,7 (years), are briefly described. It is shown that the peculiarity index, rotational velocity and magnetic field strength of all the types of CP stars do not vary during their evolution across the main sequence stripe.

Photometrically determined gravities for field and cluster Ap stars, together with published periods, show that conservation of angular momentum alone is responsible for the observed increase of the periods with age. New photometric periods of a few cluster stars are presented which independently confirm this result. No outstanding change with age of the amplitude of lightcurves is noted.

A short description of a method for analysing light curves of rotating variable stars is given. This method is applied to light curves of HD 24712.

Southern bright CP2 stars mentioned in the literature to be constant in light are observed to search for possible variations over months or years. Presently (up to end of April, 1985) four candidate long-period CP2 stars are detected, one of which had already, been noticed in the literature as a slow spectroscopic variable.

Photoelectric measurements of the β-index were made for six Ap-stars (56 Ari, 41 Tau, φ Dra, 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 increase 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.

32 stars have been checked for Δa variability, with the purpose to learn which factor(s) influence the strength of the λ 5200 continuum depression. One third of the stars appear to be constant within a range of 0.005 mag. The results on the remaining ones indicate that maximum strength is related to at least one of the magnetic poles, but spectral inhomogeneities are almost certainly responsible for the largest amplitudes.

Prom 1973 till 1981 sixteen Ap stars were investigated regarding to short time light variations with the twin telescope of the Zentralinstitut für Astrophysik stationed at the observatory Shemahka of the Academy of Science of Aserbaidshan. Only five of these stars show significant varyations with characteristical times in the region of 0.5 to 5 houres and amplitudes of about 0.01 mag. We will present here a short discussion of these five stars.

17 CP-stars have been measured in the IR filter bands J,H,K,L and M. No significant differences between CP- and normal main sequence stars can be found. Flux exzesses at 4.8 microns are not confirmed.