COMMISSION 27 OF THE I. A. U.
         INFORMATION BULLETIN ON VARIABLE STARS
                      NUMBER 282

                                    Konkoly Observatory
                                    Budapest
                                    1968 June 14


 CORRELATION OF PERIODIC VARIABILITY WITH ROTATIONAL PERIODS OF Ap STARS
              (Preliminary communication)


For 31 of the 32 Ap stars with known periods (variability of magnetic field,
spectrum and magnitude) (1), (2), (3), (6) the rotational velocity vsini was
obtained from Boyarchuk's and Kopylov's catalogue (4) or from line widths in
Babcock's catalogue (5), calibrated by stars of the catalogue (4).

If the variability is caused by rotation (e.g. oblique rotator model), the
vsini for shorter periods should be larger than for longer periods. The vsini
cannot become larger than the value corresponding to an inclination of the axis
of rotation i=90d.

The following rough estimate shows, that the distribution of inclinations of
the axis of rotation points to random distribution.

Table 1

                 Number of  From random
                  stars     distribution
    sin i>=2/3     20        22.1
2/3>sin i>=1/3      7         6.6
1/3>sin i           4         2.2

There is a good agreement qualitatively as well as quantitatively with the
results expected from the abovementioned model.

Conclusions:

1) The variability of periodic Ap variables is caused by rotation.

2) Ap stars are slower rotators than normal B and A stars,

3) The maximal radius of Ap stars is about 3 R_sun. This is in agreement with
the expected value.


                          [FIGURE 1]

  Fig.1  v.sin i versus period of variability.
         x - v.sin i from /4/, . - from line widths.
         The curves show the rotational velocities for
         rotation periods of stars with radii of 1,2,3 and
         4 R_sun.

References:

1) Kam-Ching Leung: H_gamma Intensities of Ap-Stars with known Periods; The
   Magnetic and Related Stars, p. 449, ed. by Robert C. Cameron: Mono Book
   Corporation, Baltimore, 1967
2) Observatory Reports, Lick Obs.: AJ 72, Nr. 9, 1967
3) Wehlau: PASP 74, 286, (1962)
4) Boyarchuk and Kopylow: Isw. Krimskoi Astroph. Obs. 31, 44, 1967
6) Babcock: ApJ Suppl. 3, 141, (1958)
6) Rakos: Bull. Lowell Obs. 6, No. 121, 91, (1963).


                                   WERNER SCHONEICH
                               Institut fur Sternphysik
                          Astrophysikalisches Observatorium
                                     Potsdam/DDR