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\IBVShead{4474}{30 April 1997}

\centerline{\ft THE ECLIPSING BINARY STAR MS 1428.2+0732} 
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The sky was surveyed in the X-ray region of the spectrum by the 
Einstein satellite, and the Extended Medium Sensitivity Survey 
(Stocke et al. 1991) included MS1428.2+0732 with 
the brightness given as 11.06 in V and the spectral type as F7V.
This star is also listed in the Hubble Telescope Guide Star Catalog
(GSC)(Jenkner et al., 1990) as GSC\,0331\_665.

The automated 0.5-m. telescope, Cousins R filter and CCD camera of the
Climenhaga Observatory of the University of Victoria was used to make
these photometric observations (Robb et al.  1992).  The frames had the
bias subtracted and were flat fielded in the usual manner using IRAF
\footnote{IRAF is distributed by National Optical Astronomy
Observatories, which is operated by the Association of Universities for
Research in Astronomy, Inc., under contract to the National Science
Foundation} .
The magnitudes were found from aperture photometry using the package PHOT.  
The x y pixel coordinates of each
star for photometry were found from inspection of a few frames and were used
as starting points for the Gaussian centering option which precisely
centered the 12 arc second aperture on each star for each frame. 


The primary comparison star used was SAO\,120507=GSC\,0331\_243
and the check star was GSC\,0331\_089.  The precision of the photometry 
can be estimated from the standard deviation of the differences in 
R magnitude for these two stars for each night.  This standard deviation
varies from 0.011 on a clear night to 0.033 on a poor night. 
Night to night variations can be estimated from
the mean and standard deviation of the nightly mean R magnitude
differences between the comparison and check stars.
The overall mean is $-4.100$ and the standard deviation of a night 
about this mean is $0.012$. 
The uncertainty in a measurement between the comparison
and variable star is usually smaller because the check star was
fainter.  Due to the small field of view first order extinction
effects were negligible and no corrections have been made for them.  Nor
have corrections been made for the colour difference between the stars
to transform it to a standard system. 
 
Photometric observations were begun April 1994,
continued on fifteen more nights in the spring of 1995, one night in 1996,
and one night in 1997.
Variations of brightness from night to night were soon obvious and 
the few long nights showed that the period of the variation must be 
more than a few hours. 
A sine curve was fit to various periods and
reveals a minimum average chi squared at an inverse period of
$1.21\pm0.01$ days$^{-1}$, as seen in Figure 1.  This is half the orbital 
period and other minima in the figure correspond to aliases and 
multiples of the real period.  
Times of minimum light have been found from the method of
Kwee and Van Woerden (1956) to be 2449481.7650(14), 2449499.9204(16), 
2449859.7620(5) and 2450549.7339(8) which yield a period of:
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\centerline{ HJD of Primary Minimum = 2449481.7640(6) + 1.650649(2) $\times$ E }
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\noindent where the uncertainties in the final digit are given in brackets.
These uncertainties have been underestimated, because
no allowance has been made for the asymmetry in the\break
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\centerline{2}
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\noindent  minima. A plot of the differential R magnitudes 
phased at this period is shown in Figure 2 for the data from 1994 above and 1995 below. Different runs are 
plotted with different symbols so that brightness variations from 
night to night can be seen. The 1994 data have been shifted up 0.05 
magnitudes, but the apparent difference between mean curves is about 
0.09 magnitudes, indicating that most of the light curve 
has shifted fainter about 0.04 magnitudes from 1994 to 1995. 
The bottom of primary minimum was at an intermediate level in 1996 and 1997. 

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Figure 1. Chi squared fit to a single sine curve 
for various periods 
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Figure 2.  The light curve in R for 1994 above and 1995 below
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\centerline{3}
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Figure 3.  Differential R magnitudes for Julian Date 2449832 showing a flare  
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Observations on the Julian Date 2449832 are plotted in Figure 3.  A large
flare occured at approximately 7:30 UT and lasted until 9:30 UT with an
amplitude of 0.04 magnitudes.  The peak power of the flare is of the same
order of magnitude as that of the active star RE0041+342 (Robb 1995),
which is one of the largest ever seen. 

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Figure 4.  Differential R magnitudes with example model
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The light curve modelling program Binmaker 2.0 (Bradstreet 1993) was used to 
make a light curve which approximates the data as seen in Figure 4.  The 
parameters used are temperatures of 6280 K and 3500 K, and
relative polar radii of 0.337 and 0.203 for the hot and cool star respectively.
The mass ratio was assumed to be 0.6 and the inclination was 66\degr.   
One spot was used which had a co-latitude of 60\degr, longitude
of 300\degr, radius of 21\degr\ and a temperature factor of 0.9.  All other
inputs were set at values appropriate for these temperatures. 
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\centerline{4}
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\centerline{\epsffile{4474-f5.ps}}
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Figure 5.  Scale model of the system at phase=0.75 
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A scale model of the system at phase 0.75 is shown in Figure 5 again 
produced by Binmaker 2.0 (Bradstreet 1993).  The sizes and shapes of
the stars are approximately correct for a F7V primary and a K5V secondary star.
The size and longitude of the spot are well constrained 
but the latitude of the spot is arbitrary. A better fit can be obtained by 
adding more spots, but with less confidence in their properties.

MS 1428.2+0732 is an eclipsing binary star with active regions on its
surface causing brightness variations, flares and X-ray emission from
an active corona.  Further observations will be interesting to increase
the precision of the period 
in order to look for mass transfer and magnetic braking. 

 
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\beginauthors
R.M. ROBB
Climenhaga Observatory 
of the Dept. of Physics and Astronomy
University of Victoria
Victoria, BC, CANADA,  V8W 3P6
Internet: robb@uvic.ca
\endauthors

\references

Bradstreet, D.H., 1993, Binary Maker 2.0 User Manual, Contact Software,
Norristown, PA 19401-5505, USA

Jenkner, H., Lasker, B., Sturch, C. McLean, B., Shara, M., Russell, J., 1990, 
{\it AJ}, {\bf 99}, 2082
 
Kwee, K.K. and Van Woerden, H., 1956, {\it Bull. Astr. Inst. Neth.}, {\bf 12}, 327
 
Robb, R.M.  and Honkanen, N.N.  1992, A.S.P. Conf. Ser., 28, 105, 
Automated Telescopes for Photometry and Imaging, ed. Adelman, Dukes and Adelman
%Original: {\it A.S.P. Conf. Ser.}, {\bf 38},
%Corrected: Tue Sep 27 09:13:06 MEST 2005
 
Robb, R.M., 1995, {\it IBVS}, No.\,4192
 
Stocke, J.T., Morris, S.L., Gioia, I.M., Maccacaro, T., Schild,
R., Wolter, A., Fleming, T.A., and Henry, J.P., 1991, {\it ApJ Sup}, {\bf 76}, 813 
 
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