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.

Aims. The Danish 1.54-meter telescope at the La Silla observatory was used for photometric monitoring of selected eccentric eclipsing binaries located in the Small Magellanic Cloud. The new times of minima were derived for these systems, which are needed for accurate determination of the apsidal motion. Moreover, many new times of minima were derived from the photometric databases OGLE and MACHO. Eighteen early-type eccentric-orbit eclipsing binaries were studied. Methods. Their O-C diagrams of minima timings were analysed and the parameters of the apsidal motion were obtained. The light curves of these eighteen binaries were analysed using the program PHOEBE, giving the light curve parameters. For several systems, the additional third light also was detected. Results. We derived for the first time and significantly improved the relatively short periods of apsidal motion from 19 to 142 years for these systems. The relativistic effects are weak, up to 10% of the total apsidal motion rate. For one system (OGLE-SMC-ECL-0888), the third-body hypothesis was also presented, which agrees with high value of the third light for this system detected during the light curve solution.

Context. The presence of heavier chemical elements in stellar atmospheres influences the spectral energy distribution of stars. An uneven surface distribution of these elements, together with flux redistribution and stellar rotation, are commonly believed to be the primary causes of the variability of chemically peculiar (CP) stars. Aims. We aim to model the photometric variability of the CP star ϕ Dra based on the assumption of inhomogeneous surface distribution of heavier elements and compare it to the observed variability of the star. We also intend to identify the processes that contribute most significantly to its photometric variability. Methods. We use a grid of TLUSTY model atmospheres and the SYNSPEC code to model the radiative flux emerging from the individual surface elements of ϕ Dra with different chemical compositions. We integrate the emerging flux over the visible surface of the star at different phases throughout the entire rotational period to synthesise theoretical light curves of the star in several spectral bands. Results. The synthetic light curves in the visible and in the near-UV regions are in very good agreement with the observed variability of the star. The lack of usable far-UV measurements of the star precludes making any conclusions about the correctness of our model in this spectral region. We also obtained 194 new BVRI observations of ϕ Dra and improved its rotational period to P = 1. d 716500(2). Conclusions. We show that the inhomogeneous distribution of elements, flux redistribution, and rotation of the star are fully capable of explaining the stellar variability in the visible and the near-UV regions. The flux redistribution is mainly caused by bound-free transitions of silicon and bound-bound transitions of iron.

Context. Recent statistical studies prove that the percentage of RR Lyrae pulsators that are located in binaries or multiple stellar systems is considerably lower than might be expected. This can be better understood from an in-depth analysis of individual candidates. We investigate in detail the light time effect of the most probable binary candidate TU UMa. This is complicated because the pulsation period shows secular variation. Aims. We model possible light time effect of TU UMa using a new code applied on previously available and newly determined maxima timings to confirm binarity and refine parameters of the orbit of the RRab component in the binary system. The binary hypothesis is also tested using radial velocity measurements. Methods. We used new approach to determine brightness maxima timings based on template fitting. This can also be used on sparse or scattered data. This approach was successfully applied on measurements from different sources. To determine the orbital parameters of the double star TU UMa, we developed a new code to analyse light time effect that also includes secular variation in the pulsation period. Its usability was successfully tested on CL Aur, an eclipsing binary with mass-transfer in a triple system that shows similar changes in the O-C diagram. Since orbital motion would cause systematic shifts in mean radial velocities (dominated by pulsations), we computed and compared our model with centre-of-mass velocities. They were determined using high-quality templates of radial velocity curves of RRab stars. Results. Maxima timings adopted from the GEOS database (168) together with those newly determined from sky surveys and new measurements (85) were used to construct an O-C diagram spanning almost five proposed orbital cycles. This data set is three times larger than data sets used by previous authors. Modelling of the O-C dependence resulted in 23.3-yr orbital period, which translates into a minimum mass of the second component of about 0.33 M . Secular changes in the pulsation period of TU UMa over the whole O-C diagram were satisfactorily approximated by a parabolic trend with a rate of −2.2 ms yr −1 . To confirm binarity, we used radial velocity measurements from nine independent sources. Although our results are convincing, additional long-term monitoring is necessary to unambiguously confirm the binarity of TU UMa.

This paper presents an extensive overview of known and proposed RR Lyrae stars in binaries. The aim is to revise and extend the list with new Galactic field systems. We utilized maxima timings for eleven RRab type stars with suspicious behaviour from the GEOS database, and determined maxima timings from data of sky surveys and our own observations. This significantly extended the number of suitable maxima timings. We modelled the proposed Light Time Effect (LiTE) in O−C diagrams to determine orbital parameters for these systems. In contrast to recent studies, our analysis focused on decades-long periods instead of periods in the order of years. Secondary components were found to be predominantly low-mass objects. However, for RZ Cet and AT Ser the mass of the suspected companion of more than one solar mass suggests that it is a massive white dwarf, a neutron star or even a black hole. We found that the semi-major axes of the proposed orbits are between 1 and 20 au. Because the studied stars belong to the closest RR Lyraes, maximal angular distances between components during orbit should at least be between 1 and 13 mas and this improves the chance to detect both stars using current telescopes. However, our interpretation of the O−C diagrams as a consequence of the LiTE should be considered as preliminary without reliable spectroscopic measurements. On the other hand our models give a prediction of the period and radial velocity evolution which should be sufficient for plausible proof of binarity.