COMMISSION 27 OF THE I. A. U.
            INFORMATION BULLETIN ON VARIABLE STARS
                         Number 3373

                                          Konkoly Observatory
                                          Budapest
                                          14 September 1989

                                          HU ISSN 0374 - 0676


                       THE PULSATOR 21 COM


Short-time variations in the light flux of 21 Com (V=5.46, Sp=A2p) with periods
ranging from 30 to 39 min have recently been reported by many authors (see
Weiss, 1983 and references therein). Musielok and Kozar (1982) found
variations with periods around 6 min and no evidence of other periodicities.
However the accuracy of their determination was questioned by Kurtz (1983) on
the basis of an insufficient time resolution of their data. Periods of 6 and
24 min were subsequently reported by Garrido and Sanchez-Lavega (1983). The
most recent information on the power spectrum of 21 Com was presented by Weiss
(1983). He was unable to detect any variability with periods ranging from 6 to
30 min.

The need to clarify these discrepancies on the photometric variation periods of
21 Com has led us to carry out high-speed photometric observations of this
star. The observations were carried out during 5 nights in the period February
- April, 1987 using the cooled twin-beam photoelectric photometer "URSULA",
(see De Biase et. al., 1988, for a complete description of the instrument) fed
by the 91 cm Cassegrain telescope of Catania Astrophysical Observatory. We
adopted an integration time of 20 sec, so that a total of 2598 data points,
covering about 16 hours of observation, were collected. A neutral filter was
used in order to reduce the flux coming from the bright star.


                            [FIGURE 1]

 Figure 1 - Normalized power spectra derived from the data on each 
 observation night. The spectra of the individual nights show a peak at a 
 frequency close to 0.46 mHz (36 min).

                            [FIGURE 2]

 Figure 2 - Normalized power spectrum of all data sets obtained at 
 Catania Observatory in the period February-April, 1987. The dominant 
 frequency is 0.46 mHz (36.08 +- 0.05 min). At 3 mHz no significant power 
 is apparent.


We performed a frequency analysis for each night, using the technique of
Fourier transform for unequally spaced data (Deeming, 1975). Some of the
normalized power spectra obtained for three different nights are shown in
Fig. 1. The dominant periods are close to 36 minutes and coincide, within the
errors, for all the nights.

As a next step in the frequency analysis of 21 Com data, we processed together
all the data sets. In Fig. 2 the power spectrum in the range from 0 to 0.58 mHz
is shown; it exhibits a significant peak at frequency 0.46 mHz (36.08+- 0.05
min), which corresponds, within the errors, to the values derived from each
single observation night. Moreover, the power spectra shown in Fig. 1 and in
Fig. 2 indicate that no significant frequency appears at 3 mHz; this result is
not in agreement with the conclusions of Musielok and Kozar (1982) and Garrido
and Sanchez-Lavega (1983) and suggests that 21 Com is not a rapidly oscillating
Ap star.


G. SANTAGATI (1), M. RODONO' (1), R. VENTURA (1), E. PORETTI (2)
(1) Catania Astrophysical Observatory and Astronomical
    Institute of Catania University, 95125, Catania, ITALY
(2) Merate Astronomical Observatory, 22055, Merate (Co),
    ITALY

References:

De Biase, G.A., Paterno', L., Fedel, B., Santagati, G., Ventura, R.: 1988,
   in Second Workshop on Improvements to Photometry, NASA Conference
   Publication, 10015 p. 17-33. [BIBCODE 1988itp..work...17D ]
Deeming, T.J.: 1975, Astrophys. Space Sci. 36, 137. [BIBCODE 1975Ap&SS..36..137D ]
Garrido, R., Sanchez-Lavega, A.: 1983, I.B.V.S. No. 2368.
Kurtz, D.W.: 1983, I.B.V.S. No. 2285.
Musielok, B., Kozar, T.: 1982, I.B.V.S. No. 2237.
Weiss, W.W.: 1983, Hvar Obs. Bull. 7, 263. [BIBCODE 1983HvaOB...7..263W ]