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\IBVShead{4221}{1 August 1995}

\centerline{\ft OPTICAL OBSERVATIONS OF THE ACTIVE STAR RE J2131+233 } 
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The sky was surveyed in the extreme ultraviolet (EUV) region of the
spectrum by the EUVE satellite (Malina et al., 1994) and the ROSAT
satellite (Pounds et al.,  1993) and catalogs of the sources included RE
J2131+233 = EUVE J2131+233 = BD\,+22\degr 4409 = VVO 163.  The
brightness given was 9.25 in V and the spectral type was K8 (Bowyer et
al., 1995).  The star was the subject of an extensive investigation by
Jeffries et al. (1994), however there remained a slight doubt as to the
period of the photometric variations. 

The automated 0.5-m. telescope, Johnson V filter and CCD camera of the
Climenhaga Observatory of the University of Victoria (Robb et al.  1992)
was used to make these photometric observations.  The frames were bias
subtracted and flat fielded in the usual manner using IRAF$^1$.  The
magnitudes were found from aperture photometry using the PHOT package. 
The x y pixel coordinates of each star for photometry were found from
inspection of a few frames taken at the beginning, middle and end of the
night.  These positions were used as starting points for the Gaussian
centering option which precisely centered the 6 arc second aperture on
each star for each frame. 

From the Hubble Space Telescope Guide Star Catalog (Jenkner et al., 1990)
the coordinates of the comparison star are RA=21\fh 31\fmm 07\fs\ 
Dec=23\degr 18'01'' V=10.7 and check star are RA=21\fh 30\fmm 55\fs\ 
Dec=23\degr 22'31'' V=11.5.  The mean and standard deviation of all the
nightly mean differential V magnitudes are $-1.008\pm0.007$ ensuring the
constancy of both comparison and check stars at this level.  The
precision of the differential variable star minus comparison star
measurements are expected to be at this level.  Due to the small field
of view first order extinction effects were negligible and no
corrections have been made for them.  No corrections have been made for
the colour difference between the stars to transform it to a standard
system. 
 
Photometric observations were begun 11 July 1995 UT and continued on
five more nights in the next week.  A ``Phase Dispersion Minimization"
routine modelled after that of Jurkevich (1971) reveals a minimum
average sigma at a period of $0.4232\pm0.0075$ days as seen
in Figure 1.  A least squares fit of a single sine wave to the data also
shows the deepest minimum at a period of $0.4233$.  Times of maximum
light have been found from the method of Kwee and Van Woerden (1956) to
be 2449914.8908 and 2449917.8546; and a time of minimum light to be
2449913.8295 with a formal error of about $\pm0.0008$ and an uncertainty
due to asymmetry in the extrema of about $\pm0.008$.  So the best
ephemeris from our data is:
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\centerline{ HJD of Maxima = 2449909.8059(33) + 0.4236(4) $\times$ E }
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\noindent
where the uncertainties in the final digit are given in brackets.  This 
is in agreement with the $10.17$ hour period found by Jeffries et al. (1994).
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\normalsize
$^1$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. 
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\centerline{2}
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\centerline{Figure 1. Average of standard deviations for various periods for 
RE\,J2131+233 for 1995}
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\epsffile{4221-f2.ps}
\centerline{Figure 2. Light curve of 1995 V band data}
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A plot of the 633 differential V magnitudes for the 1995 data phased at
this period is shown in Figure 2.  The light curve is almost the same
shape as that of Jeffries (1994), but the amplitude has increased from
$0.15$ to $0.20$.  Close inspection shows that the maxima repeat but one
minimum lies $0.03$ magnitudes fainter than the other observed two days
later.  This implies that the spots are changing in size and/or shape
on a daily timescale.
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\centerline{3}
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Our large data sets completely eliminate the possibility of the $9.22$
hour alternative period found by Jeffries et al.  (1994) and confirms
their $10.17$ hour period.  RE\,J2131+233 is a variable star with active
regions on its surface causing brightness variations with a rotation
period of $0.4236$ days. 

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

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\references

Bowyer, S., Lampton, M., Lewis, J., Wu, X., Jelinsky, P., Saba, V., 
Drake, J.J., Lieu, R., Malina, R.F., 1995, {\it ApJS} preprint

Jeffries, R.D., Byrne, P.B., Doyle, J.G., Anders, G.J., James, D.J, 
Lanzafame, A.C., 1994, {\it MNRAS}, {\bf 270}, 153


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

Jurkevich, I., 1971, {\it Ap. \& Sp. Sci.}, {\bf 13}, 154
  
Kwee, K.K. and Van Woerden, H., 1956, {\it Bull. Astr. Inst. Neth.}, {\bf 12}, 327

Malina, R.F., et al., 1994, {\it AJ}, {\bf 107}, 751

Pounds, K.  A., et al., 1993, {\it MNRAS}, {\bf 260}, 77
 
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: A.S.P.  Conf.  Ser.  38,
%Corrected: Mon Sep 26 13:10:13 MEST 2005
   
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