COMMISSION 27 OF THE I. A. U.
         INFORMATION BULLETIN ON VARIABLE STARS
                     Number 2368

                                    Konkoly Observatory
                                    Budapest
                                    12 July 1983

                                    HU ISSN 0374-0676


         CONFIRMATION OF THE 6 MINUTE PERIOD FOR 21 Com


  In a recent note Musielok and Kozar (1982) claimed that the light variation
of 21 Comae Berenices had a period of about 6 minutes.
  After this note, Kurtz (1983) reminded us that information cannot be 
extracted from equally spaced data beyond the Nyquist frequency.
  In June 1982 we got some measurements of 21 Com which showed a period of
6 minutes, but with a Nyquist frequency lower than 240 cd^-1 . Although the
significance of the Nyquist frequency for unequally spaced data is not 
sufficiently clear we decided not to publish these results.
  The new data, obtained in April 1983, confirm our early assumption and
that of the Musielok and Kozar that 21 Com pulsates at a frequency of about
244 cd^-1.
  To see this, we have plotted the data in Figure 1 corrected only for 
atmospheric extinction, from a set taken at Calar Alto Observatory with an
ultraviolet filter plus a neutral filter and using the 1.23 m telescope.

                        [FIGURE 1]

  The subsequent power spectrum, in Figure 2, shows a period of about 6
minutes and, also, another one of about 24 minutes which clearly appears in
Figure 1, if we look carefully at the plot.

                        [FIGURE 2]

 Apparently, the ratio between the two frequencies is four and the amplitude
of the higher frequency seems to be modulated by the period of 24 minutes.
 More observations are being carried out in order to clarify the light
curve of 21 Com, which behaves like the recently named "Rapidly oscillating
Ap stars" (Kurtz, 1982).








R. GARRIDO                 A. SANCHEZ-LAVEGA
Instituto de Astrofisica   Centro Astronomico Hispano-
de Andalucia. Apdo. 2144   Aleman. Apdo. 511
Granada. Spain.            Almeria. Spain.





References:
Kurtz, D.W., 1982, Mon. Not. R. astr. Soc., 200, 807. [BIBCODE 1982MNRAS.200..807K ]
Kurtz, D.W., 1983, I.B.V.S., No. 2285.
Musielok, B. and Kozar, T., 1982, I.B.V.S., No. 2237.