COMMISSION 27 OF THE I. A. U.
                   INFORMATION BULLETIN ON VARIABLE STARS
                               Number 3578

                                                         Konkoly Observatory
                                                         Budapest
                                                         15 March 1991
                               
                                                         HU ISSN 0374 - 0676


                  CCD PHOTOMETRY OF BX PEGASI IN 1990

Seven hundred and thirty-four CCD images of the W UMa type star BX Peg were
obtained by the authors during the Fall of 1990 using the U. S. Naval
Observatory 0.61-meter Cassegrain telescope at Washington, DC and a
Metachrome-II coated Thomson CSF THX31156 1Kx1K CCD. The effective field of
view is approximately eight arc minutes square at prime focus. A "wide red" (H
alpha-H beta-calcium) filter was employed for 706 images and "v" (GG495(2.6mm)
+ BG39(1.8mm)) was used for 28 images.

The DAOPHOT photometry package, Stetson (1987), was used in synthetic aperture
mode for five of the brightest stars in the CCD field. Differential instrumental
magnitude light curves were constructed for each nights run. The method of Kwee
and van Woerden (1956) was used to determine the times of minima. Ephemerides
given by Samec (1990) produced light curves where the primary minima were
shifted approximately -0.1 phase. An O-C diagram based on Samec's linear
ephemeris and table of times of minima and our time of minima reveals an
apparent instantaneous period change which occurred near Julian Date 2446700.0.
Photoelectric and CCD times of primary minima from Samec and wide red filter
primary times from this study were used in a linear least squares fit to
improve the ephemerides. The following new linear ephemerides were found for
the associated time intervals.

     2443790 to 2446700
     JD Hel Min I = 2445651.32225 + 0.28042068 * E                 (1)
                             +-28          +-6

     2446700 to present
     JD Hel Min I = 2448174.53105 + 0.28041747 * E                 (2)
                             +-31         +-10


Figure 1 is the "wide red" filter differential magnitudes versus phase computed using
Equation 2. Cycle to cycle variations in the light curve are evident. Table 1
gives the times of primary and secondary minima found in this study reduced to
Equation 2. Figure 2 is a plot of the O-C's of all known photoelectric and CCD
determined minima reduced using Equation 2. Minima given in Samec are denoted
with asterisks and minima from this work with filled circles. Secondary minima
were shifted by one half cycle for plotting. Lines indicate the two periods
over the interval of applicability.


                                Table 1.

    New observed epochs of primary and secondary minima fit to Equation 2.


                            PRIMARY MINIMA

                            Mean Error                O-C
Filter            HJD         (Days)       Cycles    (Days)

"wide red"   2448174.5303    0.000119        0.0    -0.0008
"wide red"   2448213.5094    0.000113      139.0     0.0003
"wide red"   2448225.5672    0.000098      182.0     0.0002
"v"          2448225.5681    0.000697      182.0     0.0011



                         SECONDARY MINIMUM

                            Mean Error                O-C
Filter            HJD         (Days)       Cycles    (Days)

"wide red"   2448191.49887   0.000151       60.5     0.0026


                           [FIGURE 1]
                           [FIGURE 2]



                                         James A. DeYoung
                                         Richard E. Schmidt
                                         Larry I. Gritz

                                         U. S. Naval Observatory
                                         34th & Mass. Ave. NW
                                         Washington DC 20392 USA



References:

Kwee, K.K. and van Woerden, H., 1956, Bull. Astron. Netherlands 12, 327. [BIBCODE 1956BAN....12..327K ]
Samec, R.G., 1990, Astron Journ. 100, 808. [BIBCODE 1990AJ....100..808S ]
Stetson, P.B., 1987, Publ. Astron. Soc. Pacific 99, 191. [BIBCODE 1987PASP...99..191S ]