COMMISSIONS 27 AND 42 OF THE IAU
              INFORMATION BULLETIN ON VARIABLE STARS
                           Number 4022

                                                 Konkoly Observatory
                                                 Budapest
                                                 10 May 1994

                                                 HU ISSN 0374 - 0676


            FURTHER CONFIRMATION THAT THE Be STAR 27 CMa
                      IS A BETA CEP VARIABLE


The Be star 27 CMa (B3IIIe) was observed intensively by Balona et al. (1987)
through the Stromgren b filter during 1986 and 1987. At that time the only
period that was present was P = 1.257 d which is typical of a periodic Be star
(lambda Eri variable). The star was observed again in 1990 when a new short-period
variation of P = 0.0918 d was discovered. The new period was still present in
1991 (Balona & Rozowsky 1991). The discovery of the onset of what appears to
be beta Cephei pulsations in a Be star is unique in two ways: 27 CMa it is the only
beta Cep star in which pulsations have been observed to begin and it is the only Be
star which shows such short-period variations.
  Pulsations in beta Cep stars are driven by the ionization zone of metals, but we
do not understand how pulsations can be excited so rapidly nor the more gradual
decrease in amplitude seen in a few beta Cep stars. Clearly, it is desirable to
monitor 27 CMa from time to time to detect any change in amplitude and to
determine whether these changes might be correlated with the mean brightness of
the star as suggested by Balona & Rozowsky (1991).
  We observed 27 CMa from two sites, CTIO (KK) and SAAO (LAB), during
3-9 January 1994. The observations at CTIO were made with the 0.6-m Lowell
telescope through the Johnson V filter and a 3.7-mag neutral density filter. The
observations at Sutherland were made with the 0.5-m telescope through the
Stromgren y filter and a neutral density filter. We used HR 2734, HR 2741 and
HR 2756 as comparison stars. The rms error of a single observation of the
comparison stars is 3 mmag; all of them were found to be constant.
  We obtained 89 V observations for 27 CMa from the two sites. The mean
magnitude was V = 4.45 which is approximately equivalent to b = 4.39.
Comparison with Balona & Rozowsky (1991) shows that the mean magnitude of
27 CMa during early 1994 was about the same as in 1991. In Fig. 1 we show
the periodogram of these data as well as the periodograms of the 1990 and 1991
data for comparison. The semi-amplitude in 1994 is 4.3 mmag which is much
the same as in 1991 but perhaps slightly higher than in 1990. Fig. 2 shows the
phase diagram with P = 0.0918827 d and epoch of phase zero at JD 2446000.000.
This period may differ from the true value by one or more yearly aliases. The
long lambda Eri period of 1.257 d is probably still present in 1994, but our data set is
not sufficient to verify this.

                             [FIGURE 1]

  Figure 1: Periodograms of 27 CMa. Frequency is in d^-1, amplitude in mmag.

                             [FIGURE 2]

  Figure 2: The light curve of 27 CMa. The magnitude scale is in mmag.


27 CMa is a close optical double with a difference in magnitude not
exceeding 0.5 mag. It is therefore possible that the beta Cep pulsations do not
originate in the Be star itself, but this does not explain the sudden visibility of
the pulsations. It has often been suggested that Be stars and beta Cep variables are
mutually exclusive. Yet most of the Be stars lie inside the beta Cep instability
strip and unless a hidden factor is at play, many of them should be beta Cep
pulsators. Rotation could be the hidden factor which suppresses pulsations, as
seems to be the case in some other types of variables (delta Scuti and 53 Per stars
for example). With v sin i = 139 km s^-1, 27 CMa is not a particularly rapid
rotator, so this possibility may still be true. If rapid rotation does suppress
pulsations in these stars, however, it calls into question the idea that low-degree
nonradial pulsation is responsible for the periodic light variations seen in a large
fraction of Be stars (lambda Eri variables).


                  L.A. BALONA
                  South African Astronomical Observatory
                  P.O. Box 9, Observatory 7935, Cape Town
                  South Africa


                  K. KRISCIUNAS
                  Joint Astronomy Centre
                  660 N. A'ohoku Place, University Park,
                  Hilo, Hawaii 96720
                  USA


References


Balona, L.A., Marang, F., Monderen, P., Reitermann, A. and Zickgraf, F.-J.:
  1987, Astr. Astrophys. Suppl. 71, 11. [BIBCODE 1987A&AS...71...11B ]
Balona, L.A. and Rozowsky, J.: 1991, Mon. Not. Roy. Astr. Soc. 251, 66p. [BIBCODE 1991MNRAS.251P..66B ]