COMMISSION 27 OF THE I. A. U.
         INFORMATION BULLETIN ON VARIABLE STARS
                      Number 3326

                                         Konkoly Observatory
                                         Budapest
                                         12 May 1989

                                         HU ISSN 0374 - 0676


       PHOTOMETRIC OBSERVATIONS OF THE Be STAR ZETA TAURI


The understanding of the physical behaviour of Be stars, from many points of
view not yet clarified, is connected to the careful observations of single
objects, during a time long enough to study quiescent and active phases.  In
this framework, we devoted much effort to the bright Be star zeta Tauri 
(HR 1910), collecting in these years a lot of spectroscopic and photometric data.
Some spectroscopic conclusions, related to the period Jan 17-24, 1983, are
reported in Guerrero et al.(1989); here we present the preliminary results
about the photometric behaviour of this intriguing star.

zeta Tauri is a well-known binary system with P=133.1d (Jarad, 1987), which
displaied both long-term shell instabilities (Delplace and Chambon, 1976) and
short-term shell and photospheric variations (Guerrero et al., 1989). As
regards the photospheric observations, several authors found brightness
variations with amplitudes from some hundredths to about a tenth of magnitude
(see, for example, Guo and Huang, 1986; Harmanec et al., 1980; Alvarez and
Schuster, 1981). In particular, Bozic (1989) claims that the light variation
time scale of zeta Tauri covers a range from several decades to few hours and
that the periods for the rapid brightness variations are equal to 0.8d and
1.6d.

We observed zeta Tauri with a digital photon-counting photometer attached to
the 50cm reflector telescope of Merate Observatory, using Stromgren y and
beta-narrow filters. Comparison and check stars were HR 1860 and 121 Tauri
respectively. The observations were obtained in the period Jan 26-Feb 15,
1989: the excellent weather allowed us to collect 491 and 388 values of Delta
m (in the sense m_HR1860 - m_zeta_Tauri) for y and beta-narrow filters
respectively.

                              [FIGURE 1]


Figure 1 represents the Delta m values in y filter versus Julian Days. As one
can see right away, an oscillation with high amplitude (about 0.4m) and long
period (several days) coexists with a shorter period and lower amplitude
variation.

This lot of photometric data, besides very close in time, allowed us to perform
a careful research of the possible periodicities. To this end, we used a
method similar to that proposed by Vanicek (1971). The y and beta-narrow data
give the same values of the periods, except P=1.416d, derived from the
beta-narrow data. Table 1 summarizes the found frequencies nu, with the
oscillation amplitudes a and the phases phi referred to t_0=7564.7936, together
with the related errors for the two filters. At last, Figure 2 represents the
Delta m(y) data with the synthesized light curve obtained for the six
frequencies shown in Table 1.

                               Table 1

           nu(c/d)                   a (mag)                    phi
                   y   beta-     y       beta-narrow      y        beta-narrow
                       narrow
     (d)
0.043(P=23.256) +-.001 +-.029 0.106 +-.003 0.088 +-.060 -1.96+-.02 -1.92+-.25
0.108(P=9.259)    .001   .009 0.058   .002 0.062   .014   .22  .03   .01  .26
0.944(P=1.059)    .001   .003 0.055   .003 0.053   .006   .90  .05   .90  .10
0.314(P=3.185)    .002   .011 0.023   .003 0.017   .006  -.26  .11  -.18  .29
2.435(P=0.411)    .011   .001 0.011   .001 0.008   .002   .61  .13   .41  .32
0.706(P=1.416)    .010   .002 0.010   .002 0.009   .005 -1.97  .26 -1.29  .66



MICHELE BOSSI, GIANANTONIO GUERRERO
Osservatorio Astronomico di Brera
via E.Bianchi 46
22055 Merate (Como)
ITALY

References:

Alvarez, M., Schuster, W.J.: 1981, Rev. Mexicana Astron. Astrof., 6, 163 [BIBCODE 1981RMxAA...6..163A ]
Bozic, H.: 1989, private communication
Delplace, A.M. and Chambon, M.T.: 1976, "Be and shell stars", ed. A. Slettbak,
   Dordrecht, Reidel, p. 79 [BIBCODE 1976IAUS...70...79D ]
Guerrero, G., Bossi, M., Scardia, M.: 1989, Astrophys. Space Sci., in press [BIBCODE 1989Ap&SS.161...27G ]
Guo, Z. and Huang, L.: 1986, 2nd Japan-China Workshop on "Stellar Activities
   and Observational Techniques", Kyoto, p. 35 [BIBCODE 1986saot.conf...35G ]
Harmanec, P., Horn, J., Koubsky, P., Zdarsky, F., Kriz, S., Pavlovski, K.:
   1980, Bull. Astron. Inst. Czechoslov., 31, 144 [BIBCODE 1980BAICz..31..144H ]
Jarad, M.M.: 1987, Astrophys. Space Sci., 139, 87 [BIBCODE 1987Ap&SS.139...83J ]
Vanicek, P.: 1971, Astrophys. Space Sci., 12, 10 [BIBCODE 1971Ap&SS..12...10V ]