COMMISSION 27 OF THE I. A. U.
             INFORMATION BULLETIN ON VARIABLE STARS
                         Number 2449

                                            Konkoly Observatory
                                            Budapest
                                            20 December 1983

                                            HU ISSN 0374-0676




                  X-RAY VARIABLE STARS IN THE PLEIADES



  In the course of an x-ray survey of the Pleiades region (Micela et al.,
1983), we have studied two x-ray exposures taken a day apart, respectively
on February 7 and 8, 1981, with the Imaging Proportional Counter aboard the
Einstein Observatory (Giacconi et al., 1979). Each exposure covers a field of
view of ~ 60'x60' with a ~ 50% overlap, 35 cluster members (m_v < 14) fall in
the useful field of view of the overlap region according to the optical catalog
of Hertzsprung (1947).
  The characteristics of both observations are summarized in Table I.


                              Table I
               Characteristics of the observations
1st Exposure (15457)                           2nd Exposure (15458)
-------------------------------------------------------------------
Start time            JD 2444642.7571           JD 2444643.9195
Effective exposure    2931.0 sec                4511.5 sec
Limiting sensitivity ~2.8 x10^29erg sec^-1     ~2.3 x10^29erg sec^-1
--------------------------------------------------------------------

  We have detected variability at a level greater than 3 sigma in the x-ray flux
of the star HzII 303, the star HzII 193 shows a marginal variation that if
real should be at the 80% level. For the remaining stars in the overlap
region counting statistics is too low to allow any positive statement on their
variability.
  The values of x-ray broad band (0.2 - 3.5 keV) count rates obtained in
the two exposures for these two stars, together with the derived fractional
variability and the level of significance are shown in Table II.

                      Table II

  Pleiades stars identified as variable x-ray sources
        1st Observ.   2nd Observ.
--------------------------------------------------------
Star#  Sp Rate*  +/-   Rate* +/-  % Variability  N sigma
--------------------------------------------------------     
193    G7  7.9   2.6   14.3  4.0       80        1.4
303    G9 12.0   3.9   37.9  6.2      177        3.0
--------------------------------------------------------

* Count-rate in units count s^-1 x 10^-3

   In the course of an independent optical survey of the Pleiades region the
star HII 303 had been observed by van Leeuwen (1983), who reports anomalies
in the V and (B-V) values and conjectures that this star MIGHT be double and
highly reddened.
  This work has been supported by NASA grant NAGW-112, and by PSN and IAIF-
CNR (ITALY). One of the authors (S. Serio) wishes also to acknowledge the support
of the Smithsonian Visitors Program.





         S. SCIORTINO, G. MICELA, S. SERIO*, G.S. VAIANA*,

                    Osservatorio Astronomico
            Palazzo dei Normanni, 90134 Palermo, Italy

          L. GOLUB, F.R. HARNDEN Jr. AND R. ROSNER

           Harvard-Smithsonian Center for Astrophysics
           60 Garden St., Cambrigde, Ma 02138, USA




* Also at Harvard-Smithsonian Center for Astrophysics





References:

Giacconi, R. et al., 1979, Ap.J., 230, 540 [BIBCODE 1979ApJ...230..540G ]
Hertzsprung, E., 1947, Ann. Leiden Obs., Part 1A [BIBCODE 1947AnLei..19A...1H ]
Micela, G.et al., 1983, 3rd Cool Stars and Stellar System Workshop, in press 
van Leeuwen, F., 1983, Sterrewacht Leiden - Royal Greenwich Observatory,
  Ph.D. Thesis [BIBCODE 1983PhDT........33V ]