COMMISSION 27 OF THE I. A. U.
         INFORMATION BULLETIN ON VARIABLE STARS
                      NUMBER 773

                                     Konkoly Observatory
                                     Budapest
                                     1973 March 14          



    A NEW VARIABLE STAR IN THE LMC WITHIN THE ERROR BOX
              OF THE X-RAY SOURCE LMC X1


In identifying the optical counterparts of discrete X-ray
sources a knowledge  of all variable stars within the error box
is of interest. Although several criteria such as synchronous
variation at X-ray and optical wavelengths may eventually be
applied to all possible candidates, pecular photometric  (flare-
like) or colorimetric  properties (ultraviolet excess) have 
previously been considered. The error box of LMC X1 (2U 0540-69), an
area of about 80 square minutes of arc centred at:

              5h40m58s -69d48'00" (Equinox 1950)

as given in the Uhuru Catalogue  (Giacconi et al. 1972), has been
searched for  variables on 32/36-inch Baker-Schmidt plates. The
plate-filter combination used matched the standard B band (103a-O+
+BG12 + GG28), and the exposure time (30 mins) reached a limiting 
magnitude of about 18.5. A total of 42 plates taken by Dr. T.
W. Rackham at the Boyden Observatory between 15 January and 28
March 1971 were available from the Armagh Observatory collection,
and cover 24 nights during that period.
                            The error box contains
                            the irregular variables
                            HV 2764 and 2760,as well
                            as a small region of
       [FIGURE 1]
                            emission nebulosity of 30
                            Doradus. Identification
                            with 30 Dor has been
                            discounted by Leong et al.
                            (1971). The newly discovered
                            variable lies about 6
                            mins of arc south of the
                            error box, i.e. just within
                            the error region of 90
                            percent confidence, at the
                            approximate position:

      Equatorial coordinates 5h41.2m -69d54,(1950)
      Harvard coordinates X = 17030 Y = 7060
A finding chart together with the comparison stars used is given 
below. The sequence is very approximate and was extrapolated
using the photoelectric magnitudes of Walker and Morris (1968)
and the estimated ADH plate limit. The brightest star in the
field of the chart has a B magnitude of about 11.1 but appears
to be anonymous. A light curve is also shown in which the star
exhibits a range of 1.5 mags from 16.2 to 17.7, appearing usually 
bright. The variable was faint on the nights 15-16 March
and 26-28 March 1971.
   Further photometric and particularly colorimetric investigations 
might be profitable. The author is indebted to Dr. C.J.
Butler of Dunsink Observatory for drawing his attention to this
X-ray source, although he points out that variation of the source
itself is still uncertain.


Armagh Observatory
7 March 1973

                          A. D. ANDREWS




References:

Giacconi,  R., Murray, S., Gursky, H. and Kellogg, E., 1972, Ap.J.
       178, 281. [BIBCODE 1972ApJ...178..281G ]
Leong, C., Kellogg, E., Gursky, H., Tanabaum, H., and Giacconi,
       R., 1971, Ap.J. (Letters) 170, L67. [BIBCODE 1971ApJ...170L..67L ]
Walker, G.A.H., and Morris, S.C., 1968, A.J. 73, 772. [BIBCODE 1968AJ.....73..772W ]