COMMISSION 27 OF THE I. A. U.
           INFORMATION BULLETIN ON VARIABLE STARS
                        Number 3653

                                             Konkoly Observatory
                                             Budapest
                                             26 August 1991

                                             HU ISSN 0374 - 0676


   Evidence for AL Comae Berenices being a Magnetic Cataclysmic Variable


AL Com was originally suspected of being a supernova based on its large
outburst amplitude; being near 9 magnitudes. Recently, it has been shown (see
refs below) to be a cataclysmic variable (CV), and a likely candidate member of
one of the magnetic subclasses.

During April 1987 (Howell and Szkody 1988) the light curve of AL Com showed
nearly equal 0.4 mag peak-to-peak modulations with a clearly defined period of
40+-1 min. Two years later (Szkody and Howell 1989), the star was observed
again at higher time resolution. Period analysis of these data revealed a
similar period of 42+-1 min, but also showed strong harmonics at 21 and 10.5
minutes. Mukai et al (1990) obtained spectroscopy of AL Com which showed
strong Balmer emission lines, thus ruling out the possibility of AL Com being
a double degenerate with a short orbital period. It was suggested, based on
these results, that AL Com was a likely member of the DQ Her subclass of
CVs, with the ~40 min period having a likely origin as the spin period of the
white dwarf.


                             [FIGURE 1]
                             [FIGURE 2]


New data obtained by us in Feb. 1991 (when AL Com was 0.6 mags brighter in V
than our previous data), showed a different light curve. The 0.4 mag
modulation was present but at a period of 84 min, which is exactly twice that
previously seen. The power spectrum shows only this single strong period, with
only a very weak indication of any period near 40 min and no other harmonics.
Since the spin period usually dominates in the optical, we cannot reconcile
these observations with a DQ Her type system (i.e., P_orb^-1 = P_spin^-1 -
P_beat^-1), unless AL Com sometimes shows a beat period of 42 min that
dominates over a spin period of 28 min for the white dwarf.

We believe a better interpretation is that AL Com has switched from two
previous active accretion areas to one current active area; behavior often seen
in AM Her stars. These types of binaries have strongly magnetic white dwarfs
which are in synchronous rotation with the secondary and show episodes during
which one or both of the accretion poles are active. X-ray observations are
needed to completely confirm our hypothesis.

We feel that the progression of the light curve over the past 5 years and the
appearance of a single strong period at 84 minutes, twice that previously seen,
provides strong evidence for a magnetic nature for AL Com.


S. B. Howell, Planetary Science Institute, 2421 E. 6th Street, Tucson, AZ 85719 USA
P. Szkody, Department of Astronomy, University of Washington, Seattle, WA 98195 USA

References:

Howell S.B. and Szkody P.,1988, PASP, 100, 224 [BIBCODE 1988PASP..100..224H ]
Szkody P. and Howell S.B., 1989, PASP, 101, 899 [BIBCODE 1989PASP..101..899S ]
Mukai K. et al, 1990 MNRAS, 245, 385. [BIBCODE 1990MNRAS.245..385M ]