CYCLIC FREQUENCY VARIABILITY IN THE RAPIDLY OSCILLATING AP STAR HR-3831

HR 3831 is an A7p SrCrEu magnetic star with an effective temperature o f T(eff) = 8000 +/- 200 K, a radius of R = 1.9 +/- 0.1 R. and an absol ute magnitude of M(bol) = 2.0 +/- 0.3. It has a polarity-reversing mag netic field which ranges from about +780 to -740 G with the rotation p eriod of P(rot) = 2.851 982 +/- 0.000 005 d. It is a visual binary wit h a separation of 3.29 arcsec; the secondary is a G2V star. The distan ce is about 66 pc. HR 3831 is a singly periodic rapidly oscillating Ap (roAp) star which pulsates with a frequency of nu almost-equal-to 1428 muHz (P= 700 s = 11.67 min). We have 666 h of photometric observation s of HR 3831 obtained in 1981, 1985, 1986, 1990, 1991, 1992 and 1993, which show that the frequency of the pulsation mode varies cyclically with a peak-to-peak amplitude of 0.12 muHz on a time-scale of 1.6 yr. A Doppler shift of the pulsation frequency caused by binary motion is ruled out by a lack of radial velocity variation. Variations over the solar magnetic cycle have recently been detected in the frequencies of global solar oscillations, with a peak-to-peak amplitude of about 4 t imes that seen in HR 3831. This suggests that the cyclic frequency var iability in HR 3831 may be caused by a cyclic modulation of the magnet ic field. If so, this is the first evidence of magnetic variations on a time-scale other than the rotation period in the magnetic stars of t he upper main sequence. Other roAp stars also show cyclic frequency va riations, as does one ZZ Ceti star.