Periods of magnetic field variations in the Ap star γ Equulei (HD 201601)

This paper presents a series of 95 new measurements of the longitudinal (effective) magnetic field B_e of the Ap star γ Equ (HD 201601). Observations were obtained at the Coudé focus of the 1-m reflector at the Special Astrophysical Observatory (SAO RAS) in Russia over a time period of 4190 d (more than 11 yr). We compiled a long record of B_e points, adding our measurements to all published data. The time series of magnetic data consists of 395 B_e points extending for 24 488 d, I.e. over 67 yr. Various methods of period determination were examined for the case in which the length of the observed time series is rather short and amounts only to 69 per cent of the period. We argue that the fitting of a sine wave to the observed B_e points by least squares yields the most reliable period in the case of γ Equ. Therefore, the best period for long-term magnetic variations of γ Equ, and hence the rotational period, is P_rot = 35 462.5 ± 1149 d =97.16 ± 3.15 yr.