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\input ibvs2.sty
\begin{document}

\IBVShead{4610}{13 July 1998}

\IBVStitle{Photometry of the W UM\lowercase{a} System GSC\,3869\_484}

\IBVSauth{R.M. ROBB and R.D. CARDINAL}

\IBVSinsto{Climenhaga Observatory, Dept.\ of Physics and Astronomy, 
University of Victoria, Victoria, BC, Canada, V8W 3P6, 
Internet: robb@uvic.ca}

\IBVSabs{GSC 3869-484 is a W UMa binary star with deep eclipses and an orbital period of 0.268 days.}
\SIMBADobj{GSC 3869-484}
\IBVStyp{ EW }
\IBVSkey{stars: activity}
\IBVSkey{binaries: close}
\IBVSkey{binaries: eclipsing}

\begintext

The star GSC 3869\_496 (Jenkner et al.\ 1990) was found to have Ca H\&K
emission in a survey by Beers et al.\ (1996).  It was also studied by 
Stephenson (1986) as part of a search for nearby K and M dwarfs.  He
classified it as a K5 star with V=11.4. The field of that star was observed with the automated 0.5m telescope and reduced in a fashion identical to that described in Robb et al.\ (1997).

\IBVSfig{7.5cm}{4610-f1.ps}{Finder chart labeled with the GSC numbers.}


Plotted in Figure 1 is the field of stars and tabulated in Table 1
are the star's identification numbers, coordinates (J2000) and magnitudes from
the Hubble Space Telescope Guide Star Catalog (GSC) (Jenkner et al.\ 1990).
Differential $\Delta$R magnitudes are calculated in the
sense of the star minus GSC\,3869\_433.
Brightness variations during a night were measured by the standard 
deviation of the differential magnitudes, which
ranged from 0\fmm 008 for bright stars on
a good night to 0\fmm 030 for the faint stars on poor nights.
The standard deviation of the nine nightly means is a measure 
of the night to night variations. 
The run means and standard deviations were
calculated and are shown as $\Delta$R in Table 1.
The high precision of these data can be seen from the standard deviation of
the $\Delta R$ 
of GSC\,3869\_395 minus GSC\,3869\_433.
The fainter stars have the expected larger standard deviation.
The standard deviation of 0\fmm013 for GSC\,3869\_496 is 
larger than we expected but
not large enough for us to be certain there are photometric variations.  
However the star GSC\,3869\_484 had obvious variations during a night
and is thus a new variable star and we devote the rest of this paper to it.


\begin{table}[h]
\caption{Stars observed in the field of GSC\,3869\_496}
\begin{center}
\begin{tabular}{rrrrr}
\hline
GSC No. & \multicolumn{1}{c}{R.A.} & \multicolumn{1}{c}{Dec.} & \multicolumn{1}{c}{GSC} & \multicolumn{1}{c}{$\Delta$R} \\
  & \multicolumn{1}{c}{J2000} & \multicolumn{1}{c}{J2000} & \multicolumn{1}{c}{Mag.} & \multicolumn{1}{c}{Mag.} \\
\hline
3869\_496& $15\hr37\mm09\sec$&$+53\deg$19\arcm22\arcs&11.1&$-2.272\pm.013$\\ 
3869\_484& $15\hr36\mm23\sec$&$+53\deg$19\arcm11\arcs&13.7&$0.133\pm.040$\\
3869\_1192& $15\hr36\mm24\sec$&$+53\deg$18\arcm00\arcs& 15. & $1.930\pm.017$\\
3869\_433 & $15\hr36\mm41\sec$&$+53\deg$17\arcm23\arcs& 12.9 & \multicolumn{1}{c}{---} \\
3869\_395& $15\hr36\mm56\sec$&$+53\deg$16\arcm42\arcs& 13.3&$ -0.001\pm.002$\\
3869\_1406 & $15\hr37\mm01\sec$&$+53\deg$16\arcm13\arcs& 15.0 &$1.876\pm.020$\\
3869\_1186 & $15\hr37\mm15\sec$&$+53\deg$16\arcm24\arcs& 13.9 &$0.664\pm.006$\\
\hline
\end{tabular}
\end{center}
\end{table}

There is no ambiguity in the determination of the orbital period of 
GSC\,3869\_484
since five of the nights included more than half the light curve.
Using data points within 0\fday04 of the minimum,
and the method of Kwee and van Woerden (1956), 
the heliocentric Julian Dates of minimum were found and are tabulated
in Table 2.

\begin{table}[h]
\caption{Times of Minimum of GSC\,3869\_484}
\begin{center} 
\begin{tabular}{ccc}
\hline
Minimum & Minimum & Minimum\\
\hline
2450977.7701 & 2450977.9033 & 2450981.7882\\
2450984.8705 & 2450985.9425 & 2450986.8826\\
2450992.7796 & 2450992.9134 & 2450993.8528\\
\hline
\end{tabular}
\end{center}
\end{table}


A fit to these times gives the ephemeris:

\vspace {3mm}
\centerline{HJD of Minima = 2450977\fday 5005(7) + 0\fday 26805(4) $\times$
E.} 
\vspace {3mm}

\noindent where the uncertainties in the final digit are given in brackets
and the mean square error of the fit is 0\fday0012.

The differential (GSC\,3869\_484$-$GSC\,3869\_395) R 
magnitudes phased at this period are plotted in Figure 2 with different 
symbols for each of the nights.  The asymmetry in the 
maxima, is indicative of star spots distributed asymmetrically over
the surface of the star(s).   


CCD frames of the field were obtained with B and V filters 
to ascertain the temperature and brightness of the variable star. 
The star GSC\,3869\_237, 10' south of GSC\,3869\_484,  
has B and V magnitudes (Urban et al.\ 
1998) measured by the Hipparcos satellite (ESA 1997). 
Relative to this star, measurements of GSC\,3869\_484 give
V=$13.54\pm.05$ and (B$-$V)=$0.73\pm.12$ at maximum light. 
From this color we estimate the spectral class of GSC\,3869\_484
to be approximately G6 (Cousins 1981).
Its period and color are consistent with the ``period-color''
diagram of Rucinski (1997).

\vfil\eject

\IBVSfig{8cm}{4610-f2.ps}{R band light curve of GSC\,3869\_484 for 1998}
\vskip 2cm
\IBVSfig{6.5cm}{4610-f3.ps}{R band light curve (points) with example model 
(line) of the eclipsing system}

The light curve leads us to expect this to be a contact system.  
Using Binmaker 2.0 (Bradstreet 1993), an example model light curve was made,
assuming the temperature of the large star to be 5400K.
The data are best fitted with an inclination of $69\deg$, a mass ratio
of 2,  and fillout factor of 0.2.
The temperature of the small star was adjusted to 5600K
and a spot $12\deg$ in radius at a longitude of $270\deg$ was added 
to get the fit seen in Figure 3. Considering the cycle to cycle variations
seen in the light curve, this is a satisfactory fit.
The mass ratio and fillout factor are poorly determined 
but the uncertainty in the inclination is about $\pm2\deg$.  
The difference in temperature and spot diameter
are known to about $\pm10\%$.

\IBVSfig{4cm}{4610-f4.ps}{
Three-dimensional model of the contact system at phase 0.25}
 
The relative sizes and shapes of the components of the system and the spot
are shown in Figure 4, again using Binmaker 2.0 (Bradstreet 1993).

The star GSC\,3869\_484 is therefore a contact 
eclipsing system with late type components and at least one spot.
Photometric observations should be continued
to monitor light curve changes due to spot migration and period changes. 
Spectroscopic observations have been started to determine a  
spectral class for the system and to measure radial velocities to 
determine the masses and the scale of the system. 

\references

Beers, T.C., Bestman, W. and Wilhelm, R., 1994, {\it AJ}, {\bf108}, 268

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

Cousins, A.W., 1981, SAAO Circ., {\bf 6}, 4

ESA, 1997, The Hipparcos and Tycho Catalogues, ESA SP-1200

Jenkner, H., Lasker, B., Sturch, C., McLean, B., Shara, M., Russell, J., 1990,
{\it AJ}, {\bf 99}, 2082

Kwee, K.K., van Woerden, H., 1956, {\it BAN}, {\bf 12}, 327

Robb, R.M., Greimel, R., Ouellette, J.,  1997, {\it IBVS} No.\,4504

Rucinski, S.M., 1997, {\it AJ}, {\bf 113}, 407

Stephenson, C.B., 1986, {\it AJ}, {\bf91}, 144

Urban, S.E., Corbin, T.E., and Wycoff, G.L., 1998, {\it AJ}, {\bf 115}, 2161

\end{document}
