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\IBVShead{4486}{4 June 1997}

\centerline{\ft THE ECLIPSING BINARY RX J1326.9+4532} 
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\begintext

The sky was surveyed in the X-ray  region of the
spectrum by the ROSAT satellite (Voges et al., 1997) and 
catalogs of the sources included 
RX J1326.9+4532 = GSC 3460\_780 (Jenkner et al., 1990).  
A literature search using SIMBAD 
shows that the star has a large proper motion
measured (Giclas et al., 1965) to be $-$.18''/an in right ascension and $-$.20''/an in declination. The star was
one of the objects found in a survey by Beers et al. (1994), who
concluded from objective prism spectral observations that it was 
``a  faint star displaying moderate CaII H\&K emission''. 

The automated 0.5-m.  telescope, Cousins R filter and CCD camera of the
Climenhaga Observatory of the University of Victoria (Robb and Honkanen,
1992) were used to make photometric observations of RX J1326.9+4532.  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} routines the frames were de-biased and 
flat fielded, and the magnitudes were found from 6 arc
second aperture photometry after using the Gaussian centering option of
the PHOT package. 


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Figure 1.  Finder chart of the field labelled with the GSC 
numbers (Jenkner et al., 1990)
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The field of stars is shown in Figure 1 and 
their designations, coordinates (J2000) and magnitudes from
the Hubble Space Telescope Guide Star Catalog (GSC) (Jenkner et al., 1990) 
and the $\Delta$R magnitudes are tabulated in Table 1.  
The $\Delta$R differences in magnitude are found from our data in the
sense of the star minus GSC 3460\_626.  
To look for brightness variations during a night the standard deviation of
the differential magnitudes for each star during a night were calculated and 
ranged from 0\fm 006 for a bright star on
a good night to 0\fm030 for the faint stars on poor nights.  
To measure night to night variations a run mean of the seven nightly averages
was calculated and is shown\vfill \eject

\centerline{2}
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\centerline{Table 1. Stars observed in the field of RX J1326.9+4532}
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\begin{center}
\begin{tabular}{ccccccc} 
\hline\\ 
 GSC No. & RA  & Dec. & GSC & $\Delta$R & V & $(V-R)_C$\\
         &J2000& J2000&  Mag. &    Mag. &    &         \\ 
\hline\\

3460\_780& $13\rm^{h}26\fmm 54\rm^{s}$& $+45\degr$32'50" & 12.5&variable& 12.80 & 0.88\\
3460\_601& $13\rm^{h}27\fmm 04\rm^{s}$& $+45\degr$35'30" & 13.4&$+1.966\pm.009$& 13.58 & 0.37\\
3460\_626& $13\rm^{h}27\fmm 24\rm^{s}$& $+45\degr$34'31" & 11.6&$-$&11.81&0.54\\
3460\_771& $13\rm^{h}27\fmm 21\rm^{s}$& $+45\degr$33'40" & 14.5&$+2.885\pm.019$& - & - \\ 
3460\_881& $13\rm^{h}26\fmm 55\rm^{s}$& $+45\degr$37'06" & 14.8&$+3.404\pm.048$& - & - \\
3460\_618& $13\rm^{h}26\fmm 39\rm^{s}$& $+45\degr$34'56" & 13.2&$+1.792\pm.031$& - & - \\
\hline\\
\end{tabular}
\end{center}
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\noindent  as $\Delta$R in Table 1.  
We consider GSC\,3460\_780 to be the only significantly variable star.
Due to the small field of view extinction effects were
negligible and no corrections have been made for them.  No corrections
have been made to transform the R magnitude to a standard system. 



Brightness variations in RX J1326.9+4532 were evident during a night.
A least squares fit of a single sine wave to all the the data shows 
a deep minimum in $\chi^{2}$ at a period of 0\fd18, but
a plot of the light curve shows unequal maxima which led us to double the
period.
An eclipse was also apparent with ingress and egress not resolved.
On three occasions the observations were terminated by clouds or dawn
during the eclipse, but
we could clearly see the ingress of the primary minimum 
seven times and the Heliocentric Julian Dates (2450000+) were 
550.9137, 560.0163, 560.7451, 562.9295, 570.9392, 576.7641, and 578.9498 
with an uncertainty of about 0.0008 days.
A fit to these times, corrected to mid eclipse gives the ephemeris:

\centerline{HJD of Minima = 2450550\fd9246(3) + 0\fd364095(7) $\times$ E.}

A plot of the  differential (GSC\,3460\_780$-$GSC\,3460\_626) R magnitudes phased 
at this period 
is shown in Figure 2 for our two best nights.  Clearly there is a 
difference of about 0\fm02 after the first maximum, but
no difference in the second maximum.  Most of the other nights agreed 
with the data marked with ``$+$'' symbols.


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Figure 2. R band light curve of RX1326.9+4532 for 
HJD 2450560(+) and 2450562($*$)
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\centerline{3}
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To help classify the variable star
V and R frames were obtained under photometric conditions along with 
observations of the nearby bright standard star 
HR 5112 (Moffett and Barnes, 1979).
The V band brightness and $(V-R)_C$ colors are listed in Table 1
for the three brightest stars. However great caution should be exercised
in using these data since they are derived from only one standard star and its
(V$-$R) was transformed from the Johnson system to the Cousins system
using  Taylor's (1986) equations.
While certainly not definitive these colors confirm that 
RX J1326.9+4532 is a late type (approximately M0V) star (Cousins 1981).  
A dwarf star of this color would be expected to have an absolute magnitude
of approximately V=9.0 (Allen 1976) so from our apparent magnitude 
we find the distance to be about 60 parsecs.  Combined with the 
proper motion we find a large tangential velocity of 76 km/sec.
Observations were continued in the V and R filters
through the eclipse and the depths were measured to be $0\fm 054\pm 0\fm 010$ in R and $0\fm 128\pm 0\fm029$ in V.

Since no points were observed on the descending or ascending branches of any
primary minimum, we observed with no filter and decreased the exposure time 
to 33 seconds for a repetition rate of 54 seconds. 
Still no points were seen on the
descending branch and only one possibly on the ascending branch.
The duration of the primary minimum was $0\fd0212\pm0.0006$ so the 
ratio of the radii of the two stars must be less than $0.028$.
All other eclipses were consistent with this duration.


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Figure 3. R band light curve (points) with example model (line) of M0V and 
white dwarf \end{center}

From the color information and the duration of the eclipse we can 
surmise that the primary star is a M0V and the secondary star is a
white dwarf.  
Assuming 0.70 and 0.47 solar masses and 0.63 and 0.014 solar radii
for the primary and secondary stars respectively (Allen 1976),
we find from our period and Kepler's $3^{rd}$ Law 
the relative radii of 0.29 and 0.0065. 
The temperature of the cool star was assumed to be 3480K and
from the depths of the minimum the temperature of the white
dwarf was estimated, but needed to be adjusted to 10000K.
Using these radii, masses and temperatures a model light curve was made with 
Binmaker 2.0 (Bradstreet 1993) and is shown in Figure 3.  
The inclination was adjusted to $76\degr$ to fit the data, however assuming 
different masses and radii would\break 

\centerline{4}
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\noindent require an inclination different by a few degrees.
To model the asymmetry in the maxima one spot was used 
which had a co-latitude of 130\degr, longitude of 60\degr, 
a radius of $13\degr$ and a temperature factor of 0.8.  
All other inputs were set at values appropriate for these temperatures.  
The cool star is well inside its Roche lobe and 
it is not likely we will see much evidence of mass transfer.


The star RX J1326.9+4532 is therefore one of a small group of stars 
very similar to the famous eclipsing binary V471 Tau.  
Both stars have white dwarf secondary stars and late type primary stars 
with evidence of starspots from asymmetrical and changing light curves,
and X-Ray emission.
Further photometric observations with a larger telescope
will be valuable to measure the relative radius,
the color and thus the temperature of the white dwarf star.  Spectroscopic 
observations will be important to get a good spectral class for the 
late type dwarf and radial velocities will measure the scale of the system
and the masses.  The space velocity is also of interest since the 
tangential velocity implies that the star may belong to Pop II.
RG would like to thank the Austrian Ministry of Science for financial support.


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

\references

Allen, C.W., 1976, Astrophysical Quantities, the Athlone Press, University
of London, London, UK

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

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

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

Giclas, H.L., Burnham, R. Jr., Thomas, N.G., 1965, {\it Lowell Obs. Bull.}, No.\,6, 197

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

Moffett, T.J. and Barnes, T.G. III, 1979, {\it PASP}, {\bf 91}, 180

Robb, R.M. and Honkanen, N.N., 1992, {\it A.S.P. Conf. Ser.}, {\bf 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

Taylor, B.J., 1986, {\it ApJS}, {\bf 60}, 577

Voges, W., Aschenbach, B., Boller,T., Braeuninger, H., Briel, U., Burkert, W.,
Dennerl, D., Englhauser, J., Gruber, R., Haberl, F., Hartner, G., Hasinger, G.,
 Kuerster, M., Pfeffermann, E., Pietsch, W., Predehl, P., Rosso, C.,
 Schmitt, J.H.M.M., Truemper, M. and  Zimmermann, H.-U., 1997, {\it A\&A}, preprint
 
   
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