COMMISSIONS 27 AND 42 OF THE IAU
              INFORMATION BULLETIN ON VARIABLE STARS
                           Number 3872

                                                 Konkoly Observatory
                                                 Budapest
                                                 23 April 1993

                                                 HU ISSN 0374 - 0676


          SAO 189111 IN SAGITTARIUS IS AN ECLIPSING BINARY


The variability of SAO 189111= DM -28d 16553 = HD 192825 was noticed when it
was used as a check star in a programme of asteroid photometry. We have been
unable to find any reference to the variability of this star in either the GCVS
(Kholopov 1987), NSV (Kholopov 1982) or in the SIMBAD database, and presume it
has not previously been reported. The star appears to be an eclipsing binary.

The initial observations were made with the 0.61-m f/16 reflector of the Mt
John University Observatory using a cooled EMI9558 photomultiplier tube and
Johnson-Cousins UBVRI filters. Follow-up observations were obtained at Mt John
and at the Black Birch Observatory, near Blenheim, New Zealand, the latter
using a 0.41-m f/13.5 reflector and cooled EMI 9558B tube. Comparison and
check star details are in Table 1. The journal of observations is given in
Table 2. A finder chart appears in Figure 1.

               TABLE 1: Catalogue Data
                                               Observed
               R.A. (1950)        DEC.        Magnitude   Spec.
SAO 189111     20h 14m 55.3s   -28d 17'21"     8.9- 9.2     G? Variable
SAO 189070     20h 12m 20.5s   -29d 21'29"       10.2       G5 Comparison
SAO 189004     20h 08m 50.8s   -29d 04'43"        9.5       G5 Check

               TABLE 2: Journal of Observations

                                  Number of
U.T. Date      HJD 2448800+       Measures       Filters     Observers   Place
July 24        28.0412- 28.2970   14             BV          B,K         MJUO
July 30        33.9118- 34.1731   16             BV          B           MJUO
July 31        34.9870- 35.1067   9              BV          B           MJUO
August 5       39.9574- 40.2280   14             BVRI        G           MJUO
August 17      51.8872- 52.0826   8              UBVR        G           MJUO
August 19      54.1616- 54.2104   4              UBV         G           MJUO
August 21      56.0057- 56.1282   14             UBVR        B           BBO
August 22      56.8383- 57.2058   39             UBVR        B           BBO
August 23      57.8149- 58.2061   41             UBVR        B           BBO
September 17   82.9079- 83.1283   10             UBVR        G           MJUO

Notes: Observers : B = Blow, G = Gilmore, K = Kissling.
Place :      BBO = Black Birch Observatory, 0.41 m f/13.5 reflector.
             MJUO = Mt John University Observatory, 0.6 m f/16& f/13 reflectors.


                               [FIGURE 1]

                      FIGURE 1: Field of SAO 189111

                               [FIGURE 2]

                FIGURE 2: Plot of V Magnitude versus Phase


From measure of A.W.J. Cousins's E region standards (Menzies et al, 1980) the
comparison star was found to have V =10.17, U-B = +0.06, B-V = +0.56, V-R = +
0.35, V-I = +0.68. These values were added to the standard differential
magnitudes of the variable.

On nights where variability was noted, both the V and B light curves followed
portions of a smooth decline and rebrightening covering about 0.3 magnitudes
over a period of approximately six hours. Definite minima occurred on July 24
and 30, and on August 22 and 23. Although all the minima are very similar in
shape, those observed in July are slightly wider and deeper than those observed
in August.

The observed minima in the lightcurve are best matched with a period of 1.177
days. Since minima were observed on the nights of both August 22 and 23,
periods longer than 1.177 days are not possible. Times of primary minimum
given by:

HJD (Primary Min.) = 2448834.04 + 1.177 E
                       +/- 0.01   0.001

A plot of the V data based on a period of 1.177 days appears in Figure 2. The
combined colour, data was noisy, probably due to frequently poor skies. No
convincing evidence for variation in colour from a mean B-V = 0.65 was seen.

The shape of the lightcurve and the period suggest a close eclipsing binary
system. We note that the Michigan Spectral Catalogue (Houk 1982) describes SAO
189111 as being of composite spectral types "G5/8 + A/F", consistent with this
deduction. The possible plateau feature following primary eclipse may suggest
the presence of a chromospherically active star.


Warwick M. Kissling              Graham L. Blow
Carter Observatory               Carter Observatory
P.O. Box 2909                    P.O. Box 2909
Wellington                       Wellington
New Zealand                      New Zealand

Alan C. Gilmore
Mt John University Observatory
and Department of Physics & Astronomy,
University of Canterbury
Christchurch, 1
New Zealand.

References:

Houk, Nancy (1982) Michigan Catalogue of Two-Dimensional Spectral Types for the
    HD stars, Vol 3.; University of Michigan.
Kholopov, P.N. (ed) (1982) New Catalogue of Suspected Variable Stars, Nauka,
    Moscow. [BIBCODE 1982ncss.book.....K ]
Kholopov, P.N. (ed) (1987) General Catalogue of Variable Stars, 4th Edition,
    Nauka, Moscow. [BIBCODE 1987gcvs.book.....K ]
Menzies, J.W., Banfield, R.M. & Laing, J.D., (1980). Standard Stars for
    Photoelectric Photometry on the UBV and (RI)KC Systems, SAAO, Cape Town. [BIBCODE 1980SAAOC...1..149M ]