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\centerline{\ft OPTICAL OBSERVATIONS OF THE STAR RX J1239.8+5511} 
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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 J1239.8+5511 = GSC\,3844\_317 (Jenkner et al., 1990).  


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 J1239.8+5511.  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 labeled 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) 
are given in Table 1.
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\fm004 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 fourteen nightly
averages was calculated and is shown in Table 1 as $\Delta$R in 
the sense the star minus GSC 3844\_650. 
We consider GSC\,3844\_317 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. 
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\centerline{Table 1. Stars observed in the field of RX J1239.8+5511}
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\begin{tabular}{ccccccc} \hline \hline
 GSC No. & RA  & Dec. & GSC & $\Delta$ R & V & $(R-I)_C$\\
         &  J2000.   &      J2000.    &  Mag.       &     Mag.  \\ 
\hline \\

3844\_317& $12^h39^m52^s$& $+55^o$11'21" & 10.9&variable& 11.77 & 
0.66\\
3844\_650& $12^h40^m19^s$& $+55^o$12'18" & 11.3&-& 11.71 & 0.35\\
3844\_683& $12^h39^m33^s$& $+55^o$13'47" & 
12.4&$+1.003\pm.015$&12.92&0.35\\
3844\_519& $12^h40^m04^s$& $+55^o$14'50" & 15.3&$+3.701\pm.058$& 
- & - \\ 
3844\_548& $12^h40^m26^s$& $+55^o$11'03" & 
14.1&$+2.806\pm.051$& - & - \\ 
\hline
\end{tabular}
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Figure 2. Period search of the nightly means of RX1239.8+5511.
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Figure 3. R band light curve of RX J1239.8+5511 for 1997 
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Brightness variations in RX J1239.8+5511 were barely detectable
during a night, but were obvious from night to night.
A sine curve was fit to the nightly means and the $\chi^2$ for 
various periods is shown graphically in Figure 2.  The best fit
was found for a frequency of 0.1407 cycles per day and 
semi-amplitude of 0\fm174. This gives the ephemeris: 
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\centerline{HJD of Maxima = 2450583\fd4(3) + 7\fd1(4) 
$\times$ E.}
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where the uncertainty in the final digit is given in brackets.
A plot of the nightly mean differential (GSC\,3844\_317$-$3844\_650) R
magnitudes phased at this period is shown in Figure 3 with
different symbols for each of the different nights. 



To help classify the variable star B, V, R and I frames were
obtained under photometric conditions (JD\,2450608) along with
observations of the nearby bright standard stars HR\,4660,
HR\,4716, HR\,4931 and HR\,5154 (Moffett and Barnes, 1979). The V 
magnitudes and $(R-I)_C$ colors are listed in Table 1 for the
three brightest stars. The random errors for these data are about
$0\fm03$. However great caution should be exercised in using
these data since they are derived from only a few standard stars
and their (R$-$I) was transformed from the Johnson system to the
Cousins system using the equations of Taylor (1986). While
certainly not definitive these colors confirm that RX
J1239.8+5511 is a late type (approximately K4) star (Cousins
1981).  From the admittedly poorly determined (B$-$V) of $0\fm8\pm
0\fm1$ RX J1239+5511 is more likely a dwarf and not a giant star. 
Assuming an absolute magnitude of 7\fm0 (Allen 1976) we find a 
distance of approximately 90 parsecs. 

From the shape and amplitude of the light curve and the length of the
period we would expect that this is a single K4V star with spots and
X-rays produced by an active corona. It is possible that this star is a
giant with spots or with a close companion either heating one hemisphere
or causing a tidal distortion.  To eliminate this possibility further
photometric observations are useful to look for variations in color or in
the height of the maxima.  Spectral observations would be helpful to
determine the spectral type, to look for Ca H\&K emission, to determine 
$v \, sin(i)$ and to look for radial velocity variations. 

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

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

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


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\centerline{4}
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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, A\&A, preprint
 
   
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