The record-breaking rotational deceleration of the He strong CP star HD37776 (V901 Ori)

We study the long-term light and spectral variations in the Hestrong, magnetic chemically peculiar star HD37776 to search for changes of its 1.5387 d period in 1976–2007. We analyze all published photometric observations and spectrophotometry in the He i 4026°A-line. The data were supplemented with 506 new (U)VB observations obtained during the last 2 observing seasons, 66 estimates of He i equivalent widths on 23 CFHT spectrograms and 69 on Zeeman spectrograms from the 6-m telescope. All the 1895 particular observations have been processed simultaneously. We confirm the previously suspected increase of the period which is a record among CP stars. The mean rate of the period increase during the last 31 years is 0.541±0.020 s per year. We interpret this ongoing period increase as the slowing down of the star’s surface rotation due to momentum loss through events and processes in its magnetosphere