Phase instability and non-linearity in the distorted dipole pulsation mode of the rapidly oscillating Ap star HR 3831 (HD 83368)

HR 3831 is a rapidly oscillating A7p SrCrEu star with T eff = 8000 K, R = 1.9 R ○ ., M V = 2.0, V = 6.168, d ≃ 60 pc, a magnetic field which ranges from about −720 G ≤ B eff ≤ + 780 G with the rotation period of 2.851982 d, and a pulsation period near 700 s. In this paper, new high-speed photometric observations obtained during 101 h in 1990 and 167 h in 1991 are presented. The 1991 observations were obtained contemporaneously from the South African Astronomical Observatory and Cerro Tololo Inter-American Observatory over a time-span of 17 d with a 41 per cent duty cycle