Context. Light and spectrum variations of the magnetic chemically peculiar (mCP) stars are explained by the oblique rigid rotator model with a rotation period usually assumed to be stable on a long time scale. A few exceptions, such as CU Vir or 56 Ari, have been reported as displaying an increase in their rotation period. A possible increase in the period of light and spectrum variations has also been suggested from observations of the helium-strong mCP star HD 37776 (V901 Ori). Aims. In this paper we attempt to confirm the possible period change of HD 37776 and discuss a possible origin of this change as a consequence of i) duplicity; ii) precession; iii) evolutionary changes; and iv) continuous/discrete/transient angular momentum loss. Results. We confirm the previously suspected gradual increase in the 1. d 5387 period of HD 37776 and find that it has lengthened by a remarkable 17.7 ± 0.7 s over the past 31 years. We also note that a decrease in the rate of the period change is not excluded by the data. The shapes of light curves in all colours were found to be invariable. Conclusions. After ruling out light-time effects in a binary star, precession of the rotational axis, and evolutionary changes as possible causes for the period change, we interpret this ongoing period increase as a braking of the star's rotation, at least in its surface layers, due to the momentum loss through events or processes in the extended stellar magnetosphere.

The early-type chemically peculiar stars often show strong magnetic fields on their surfaces. These magnetic topologies are organized on large scales and are believed to be close to an oblique dipole for most of the stars. In a striking exception to this general trend, the helium-strong star HD 37776 shows an extraordinary double-wave rotational modulation of the longitudinal magnetic field measurements, indicating a topologically complex and, possibly, record strong magnetic field. Here we present a new investigation of the magnetic field structure of HD 37776, using both simple geometrical interpretation of the longitudinal field curve and detailed modeling of the time-resolved circular polarization line profiles with the help of magnetic Doppler imaging technique. We derive a model of the magnetic field structure of HD 37776, which reconciles for the first time all magnetic observations available for this star. We find that the local surface field strength does not exceed ≈ 30 kG, while the overall field topology of HD 37776 is dominated by a non-axisymmetric component and represents by far the most complex magnetic field configuration found among early-type stars.

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.

Spectropolarimetric observations of 96 chemically peculiar (CP) main‐sequence stars have been carried out at the 6‐m telescope at the Special Astrophysical Observatory of the Russian Academy of Sciences (SAO RAS) with the aim of searching for the presence of stellar magnetic fields. The stars selected for investigation were CP stars known to have strong anomalies in the wavelength region of the continuum flux depression around λ 5200 Å. This selection was conducted with the aid of low‐resolution spectral observations, made with the SAO RAS 1‐m telescope, and of published differential photometric data. Magnetic fields have been successfully detected in 72 stars of which only three stars were previously known to have magnetic fields. For two stars, the longitudinal component of the magnetic field Be exceeds 5 kG: HD 178892 – 7.4 kG, and HD 258686 – 6.7 kG. We failed to reliably detect the magnetic field in the other 24 CP stars. These stars are mostly fast rotators, a feature which hampers accurate measurements of Be. It is demonstrated in this paper that selecting candidate magnetic stars by considering their photometric indices Z or Δa, or alternatively, by inspecting low‐resolution spectra around the λ 5200 Å flux depression, considerably increases the detection rate.

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