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CU Vir and V901 Ori belong among these few magnetic chemically peculiar starswhose rotation periods vary on timescales of decades. We aim to study thestability of the periods in CU Vir and V901 Ori using all accessibleobservational data containing phase information. We collected all availablerelevant archived observations supplemented with our new measurements of thesestars and analysed the period variations of the stars using a novel method thatallows for the combination of data of diverse sorts. We found that the shapesof their phase curves were constant, while the periods were changing. Bothstars exhibit alternating intervals of rotational braking and acceleration. Therotation period of CU Vir was gradually shortening until the year 1968, when itreached 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 therotation has begun to accelerate again. We also found much smaller periodchanges in CU Vir on a timescale of several years. The rotation period of V901Ori was increasing for the past quarter-century, reaching a maximum of 1.538771d in the year 2003, when the rotation period began to decrease. A theoreticallyunexpected alternating variability of rotation periods in these stars wouldremove the spin-down time paradox and brings a new insight into structure andevolution of magnetic upper-main-sequence stars.

Surprising variations in the rotation of the chemically peculiar stars CU Virginis and V901 Orionis