Context. The majority of magnetic chemically peculiar (mCP) stars exhibit periodic light, radio, spectroscopic and spectropolarimetric variations that can be adequately explained by the model of a rigidly rotating main-sequence star with persistent surface structures. CU Vir and V901 Ori belong among these few mCP stars whose rotation periods vary on timescales of decades. Aims. We aim to study the stability of the periods in CU Vir and V901 Ori using all accessible observational data containing phase information. Methods. We collected all available relevant archived observations supplemented with our new measurements of these stars and analysed the period variations of the stars using a novel method that allows for the combination of data of diverse sorts. Results. We found that the shapes of their phase curves were constant during the last several decades, while the periods were changing. At the same time, both stars exhibit alternating intervals of rotational braking and acceleration. The rotation period of CU Vir was gradually shortening until the year 1968, when it reached its local minimum of 0.52067198 d. The period then started increasing, reaching its local maximum of 0.5207163 d in the year 2005. Since that time the rotation has begun to accelerate again. We also found much smaller period changes in CU Vir on a timescale of several years. The rotation period of V901 Ori was increasing for the past quarter-century, reaching a maximum of 1.538771 d in the year 2003, when the rotation period began to decrease. Conclusions. We propose that dynamical interactions between a thin, outer magnetically-confined envelope, braked by the stellar wind, with an inner, faster rotating stellar body is able to explain the observed rotational variability. A theoretically unexpected alternating variability of rotation periods in these stars would remove the spin-down time paradox and brings a new insight into structure and evolution of magnetic upper-main-sequence stars.

Context. The spectral energy distribution (SED) in chemically peculiar stars may be significantly affected by their abundance anomalies. The observed SED variations are usually assumed to be a result of inhomogeneous surface distribution of chemical elements, flux redistribution and stellar rotation. However, the direct evidence for this is still only scarce. Aims. We aim to identify the processes that determine the SED and its variability in the UV and visual spectral domains of the heliumweak star CU Vir. Methods. We used the TLUSTY model atmospheres calculated for the appropriate surface chemical composition to obtain the emergent flux and predict the rotationally modulated flux variability of the star. Results. We show that most of the light variations in the vby filters of the Strömgren photometric system are a result of the uneven surface distribution of silicon, chromium, and iron. Our models are only able to explain a part of the variability in the u filter, however. The observed UV flux distribution is very well reproduced, and the models are able to explain most of the observed features in the UV light curve, except for the region 2000−2500 Å, where the amplitude of the observed light variations is higher than predicted. The variability observed in the visible is merely a faint gleam of that in the UV. While the amplitude of the light curves reaches only several hundredths of magnitude in the visual domain, it reaches about 1 mag in the UV. Conclusions. The visual and UV light variability of CU Vir is caused by the flux redistribution from the far UV to near UV and visible regions, inhomogeneous distribution of the elements and stellar rotation. Bound-free transitions of silicon and bound-bound transitions of iron and chromium contribute the most to the flux redistribution. This mechanism can explain most of the rotationally modulated light variations in the filters centred on the Paschen continuum and on the UV continuum of the star CU Vir. However, another mechanism(s) has to be invoked to fully explain the observed light variations in the u filter and in the region 2000−2500 Å.

Abstract. A new method of determination of the effective temperatures of Ap stars is proposed. The method is based on the fact that the slopes of the energy distribution in the Balmer continuum near the Balmer jump for "normal" main sequence stars and chemically peculiar stars with the same T eff are identical. The effective temperature calibration is based on a sample of main sequence stars with well known temperatures (Sokolov 1995). It is shown that the effective temperatures of Ap stars are derived by this method in good agreement with those derived by the infrared flux method and by the method of Stepien & Dominiczak (1989). On the other hand, the comparison of obtained T eff with T eff derived from the color index (B2-G) of Geneva photometry shows a large scatter of the points, nevertheless there are no systematical differences between two sets of the data.

The spectrophotometric variability of the magnetic chemically peculiar star 56 Arietis (56 Ari) in the far‐ultraviolet spectral region from 1150 to 1980 Å is investigated. This study is based on the archival International Ultraviolet Explorer data obtained at different phases of the rotational cycle. The brightness of 56 Ari is not constant in the investigated wavelengths over the whole rotational period. The monochromatic light curves continuously change their shape with wavelength. The comparison of energy distributions at three phases shows that the first minimum of light curves at phase 0.25 is replaced by the maximum for λ > 1608 Å, but the second minimum of light curves at phase 0.65 is absent in the spectral region between λ1938 and λ1980 Å. Iron is responsible for the depression at λ1775 Å in the spectrum of 56 Ari.