THE DETERMINATION OF THE ROTATIONAL PERIODS OF THE RAPIDLY OSCILLATING AP STARS FROM THEIR MEAN LIGHT VARIATIONS .5. AN IMPROVED ROTATION PERIOD FOR THE DIPOLE PULSATOR HD 6532

We have obtained 87 new differential UBVRI mean Light observations of the rapidly oscillating Ap star HD 6532 on 34 nights in 1987 and on 31 nights in 1994. A frequency analysis of these data, together with 43 published observations obtained on 34 nights in 1986, gives a rotation period of P-rot = 1.944 973 +/- 0.000 011 d, or a rotation frequency of nu(rot) =0.514 146 +/- 0.000 003 d(-1) = 5.950 76 +/- 0.000 03 mu H z. The mean light curve shows a clear double wave which is consistent with both magnetic poles being visible over a rotation period. The U a nd B curves are in antiphase with the V, R and I curves. The null wave length, therefore, lies between B and V. This improved rotation freque ncy is needed for future interpretation of the frequency multiplet gen erated by the rotational, modulation of the rapid 6.9-min pulsation in this star.