Frequency variability in the rapidly oscillating Ap star HR 3831: Three more years of monitoring

We present 147 h of high-speed photometric observations of HR 3831 obtained on 125 nights in 1993, 1994, 1995 and 1996. In an ongoing programme, we monitor the pulsation of this star for 1 h on each possible night from November through June to determine its pulsation phase. We then use those pulsation phases to study its frequency variability. We show the character of that frequency variability in phase O-C diagrams for the last five years, during which we have monitored the star intensively, and for all of the data available since 1980. While our previous suggestion that the frequency variability is cyclic is still viable, the character of the variability is not obvious. We rule out the possibility that the frequency variability results from Doppler shifts. We also show evidence that the pulsation amplitude varies along with the frequency. These variations are intrinsic to the star, and hence are indicative of changes in the properties of the pulsation cavity. There is no physical model to explain the observations, other than our previous speculation that they may be indicative of a magnetic cycle. We use the new observations, together with all previous observations, to refine the rotation frequency to nu(rot) = 4058.265 +/- 0.004 nHz (P-rot = 2.851976 +/- 0.00003 d), under the assumption that the oblique pulsator model is correct, We also refine the values of, and reduce the errors on, the amplitudes and phases of the frequency septuplet, the first harmonic quintuplet, second harmonic triplet and third harmonic singlet which characterize the non-linear, distorted dipole pulsation mode of this star.