COMMISSION 27 OF THE I. A. U.
           INFORMATION BULLETIN ON VARIABLE STARS
                       Number 3647  

                                           Konkoly Observatory
                                           Budapest
                                           12 August 1991

                                           HU ISSN 0374 - 0676


          ON THE STABILITY OF THE PERIOD OF BO CEPHEI


For several decennia BO Cep has been regarded as the prototype of irregular
variable stars which show Algol-like, but unperiodic, sharp fadings of their
brightness. Frequently the star was mentioned in the same breath together with
the extremely young stars of T Orionis type, but its evolutionary state has
never been firmly demonstrated.

Recently Grankin et al. (1991) showed that in their photoelectric brightness
data of the "normal light" of BO Cep a period of about 10 days was hidden, and
they constructed good-looking mean lightcurves for the four years 1987 to 1990
of their measurements.

Our own photoelectric material of 1965 to 1976 does not only confirm their
period, but, what is more, coherently obeys their elements,

              C = 244 6364.567 + 10.658d * E

Fig. 1 shows the whole of our brightness values folded with these elements.
The comparison stars used and their magnitudes are given by Wenzel and Brückner
(1978).

Additionally all but one of the deep visual and photographic minima (>=0.5 mag)
described by Hoffmeister (1944) and Bradl (1978) as well as the - though
shallow - three photoelectric fadings of Kardopolov and Shutemova (1980) and of
Kovalchuk and Pugach (1980) are characterized by O-C values in the range of
+-0.14p, provided the above given period is improved by subtracting
0.00019d and amounts to P_1 = 10.65781d. Fig. 2 shows the O-C diagram, where
dots represent photoelectrically observed fadings and crosses the deep (and
therefore sure) visual and photographic minima; all material has been taken
from the quoted papers and observational series.

Thus our observations point to a long-term existence and stability of the
period and do not contradict the assumption of Grankin et al. (l.c.) that we
deal with an orbital revolution in a binary system.


                               [FIGURE 1]
                               [FIGURE 2]


The reader must be aware of the fact that obviously the minima are of very
different depth and that moreover in numerous cases they cannot be realized at
all at the predicted dates. The model of Grankin et al. (l.c.) can account
for these findings, but a thorough spectroscopic investigation of this
important object is clearly overdue. In this connection we remind of two more
kinds of "eclipsing" stars with vanishing minima, represented by Kohoutek's
central star V 651 Mon of a planetary nebula and by SS Lac with a possibly
perturbed orbit, see Lehmann (1991). Maybe these cases are more numerous than
hitherto supposed. The large number of eclipsing variables, for which no
period could be found, could be explained in this way and must not be
necessarily due to the scarcity of the respective data, as has been assumed
frequently.


                              W. WENZEL
                              Sternwarte Sonneberg
                              Deutschland

References:
Bradl, I.: 1978, unpublished, cited by Wenzel, W., Bruckner, V. (1978).
Grankin, K.N. et al.: 1991, I.B.V.S., No. 3627.
Hoffmeister, C.: 1944, Astron. Nachr. 274, p.232. [BIBCODE 1944AN....274..232H ]
Kardopolov, V.I., Shutemova, N.A.: 1980, Perem. Zvezdy 21, p.305.[BIBCODE 1980PZ.....21..305K ]
Kovalchuk, G.U., Pugach, A.F.: 1980, Mitt. Veranderl. Sterne 8, p.129.[BIBCODE 1980MitVS...8..129K ]
Lehmann, T.: 1991, I.B.V.S., No. 3610.
Wenzel, W., Bruckner, V.: 1978, Mitt. Veranderl. Sterne 8, p.35.[BIBCODE 1978MitVS...8...35W ]