
% IBVS article on EQ Tau by Benbow and RLM - draft of April 24, 95.
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\IBVShead{4187}{3 May 1995}   % Use it with dummy or empty args

% Call for macro \title {Title of the paper}
\centerline{\ft ECLIPSE OBSERVATIONS OF EQ Tau}
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EQ Tau (G1, V=11.9, \ra[03 48 12.9], \dec[+22 18 49], J2000) is a
poorly studied W UMa eclipsing binary with a period of 0.34 days. This
system is on the AAVSO list of eclipsing binaries (Baldwin and
Samolyk 1993) but to our knowledge has had no previously published
light curve or ephemeris. The AAVSO reference reports eclipse minima
observations (chiefly by G. Samolyk) during the period JD\,2442832 (1976)
to 2448694 (1992). An O$-$C plot using the AAVSO observations indicates
that the published period of 0.34134848 days (Brancewicz and
Dworak, 1980) was slightly long, but it was not clear whether the period
was constant or whether there was a small period derivative.

The present note describes CCD photometry of EQ Tau done using the
University of Iowa Automated Telescope Facility located in Iowa City,
IA. The system consists of an 18cm refractor, a Spectrasource HPC-1 CCD
camera  (format $512 \times 512$ binned pixels, $3''\!.00$ per pixel)
and a Johnson $R$-band filter. We used the nearby GSC stars at \ra[03
48 31.3], \dec[+22 12 43], and \ra[03 48 16.2], \dec[+22 17 28], J2000)
as check and comparison stars respectively. Each observation consisted
of 45 second exposures of a field containing EQ Tau as well as the
check and comparison star. This cycle was repeated every 5 minutes for
several hours each night. The differential aperture photometry was
performed by an automated procedure after aligning all images to a
common stellar reference. No air mass or color corrections were
applied.  The system was observed during three epochs (1 Dec 1994, 8
Feb 1995, 25 Feb 1995). By combining all three epochs, we obtained the
nearly complete light curve as shown in Figure 1. The minimum at phase
0.5 occurred at heliocentric Julian date 2,449,687.607$\pm$0.007. Note
the $\sim$0.05 magnitude depression present between phase 0.7-0.85 in
both February epochs only. We believe this is not an artifact and may
have been due to an active region on the stellar surface which developed since
the December 1994 observations.

The O$-$C measurements available from the AAVSO compilation clearly
showed that the previously available ephemeris:  $$JD_{min} =
2,440,203.325+0.34134848\times E$$ was slightly in error, although it
was unclear whether the deviation could be better fit with a linear or
a quadratic dependence. The present observations are in better
agreement with a linear fit and new ephemeris given by $$JD_{min}  =
2,440,203.342 + 0.34134750(\pm 0.00000015)\times E$$ where the period
uncertainty represents the 90\% confidence bounds.  A plot of the AAVSO
observations, along with the minimum reported in this note, is shown in
Figure 2.

Interesting parties can obtain the raw photometry data from the
authors at the following e-mail addresses:  {\em rlm@astro.physics.uiowa.edu} or {\em uwb@astro.physics.uiowa.edu}.\break
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Figure 1. Light curve of EQ Tau. The depression evident
near phase 0.7-0.8 seen during February 1995 may have been caused by a starspot.
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\centerline{\epsffile{4187-f2.ps}}
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Figure 2. O$-$C plot for EQ Tau using JD=2,440,213.325+0.34134848$\cdot$N, where N is the cycle number. A least-squares linear and quadratic fit is also shown. The
linear fit represents the new ephemeris 2,440,203.342 + 0.34134750$\cdot$N. 
\end{center}
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\centerline{3}
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This research has been supported by the Iowa Space Grant Consortium and the
National Science Foundation. We have also made use of the Simbad database,
operated at CDS, Strasbourg, France.

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% Call for macros \authors - \authorsrightcol - \endauthors
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W. R. BENBOW
R. L. MUTEL
Van Allen Observatory
Dept. Physics and Astronomy
University of Iowa
Iowa City IA 52242
\endauthors

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\references

Baldwin, M. and Samolyk, G. 1993, AAVSO Observed Minima Timings of Eclipsing       Binaries No.\,1.

Brancewicz, H. and Dworak, T., 1980, {\it Acta Astron.}, {\bf 30}, 501

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