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\centerline{\ft V-BAND PHOTOMETRY OF XY UMa IN 1993 AND 1995}
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\begintext

XY UMa (= SAO\,27143 = BD+55\degr 1317, G2V + K5V) is a chromospherically
active, close, eclipsing binary system, with a period of 0.48~days.
It has been the subject of close scrutiny recently, at both optical and
X-ray wavelengths, where the eclipses can be used to spatially probe
magnetic structures on and above the surface (Bedford et al. 1990; 
Hilditch \& Bell 1994 -- hereafter HB94). 
Hilditch \& Collier Cameron (1995 -- hereafter
HC95) have developed a model to explain the changing optical light curve of XY
UMa in terms of starspots covering a wide range of latitudes on the
surface of the primary star. The most recent analysis of the O$-$C times
of minima was performed by Pojma\'{n}ski \& Geyer (1990 -- hereafter
PJ90). 
They find
evidence for a quasi-sinusoidal variation in the O$-$C's, which
can be explained if there is a third low-mass star in the system, with
an orbital period of either 25 or 40~years. Ar\'{e}valo et al. (1994)
claim to have detected a third body about 2 arcseconds away from the
close binary system. As a further contribution to the wealth of
historical data that is accumulating for this fascinating object, we
present two V-band light curves obtained in 1993 and 1995.

Both data sets were taken with a CCD camera
attached to
the University of Birmingham 0.4-m telescope. A V-band filter
manufactured by Murnaghan Instruments was used. This had a peak
throughput at 525\,nm and a FWHM of 110\,nm.
The 1993 data were taken
at the f/19 Cassegrain focus
on the nights of the 19 February, 8/9 March and 13/14 March. The 1995 data were
taken at the f/5 prime focus 
on 26/27 February, 3 March, 15 March and 17 March. 
The CCD frames were bias subtracted, flat-fielded and counts were
integrated using a 24 arcsecond radius aperture.
The comparison star used in each case was BD+55\degr\,1320AB. Both
components were included well inside the software aperture.
The derived V-band differential magnitude 
light curves are shown in Figures 1 and 2. 
Phases were calculated according to the least squares linear ephemeris
of HJD\,$2435216.5018+0.47899493E$ from PJ90.
The full heliocentric observation times and differential
magnitudes can be obtained from the authors.
Two new epochs of primary minima were derived by fitting parabolas to
the bottom half of the primary eclipses. We obtain 
HJD\,$2449055.6183\pm0.0004$ and HJD $2449775.5469\pm0.0008$ where the
quoted errors are $68\%$ confidence limits. The O$-$C values are
$-$0.0050~days and $-$0.0058~days respectively. 

A comparison with the various attempts to explain the O$-$C variations in
PJ90, reveals that neither their short or long period, circular or
eccentric third body hypotheses adequately fit our times of minima. In
both cases (and also for the times of minima in 1992 given by HB94) the
O$-$C's are significantly positive (by 0.003 to 0.006 days). 
The short period ($\sim 25$\,yrs) hypothesis is marginally worse than the
long period ($\sim 40$\,yrs) solution. A complicating factor may be
errors in the minima determinations caused by light curve asymmetries,
but both our 1993 light curve and HB94's 1992 light curve have
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\begin{center}
Figure 1. 
Phased differential V-band light curve of XY UMa taken during
February/March 1993.
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\large

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\begin{center}
Figure 2. Phased differential V-band light curve of XY UMa taken during
February/March 1995.
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\large
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Figure 3.
Historical brightness of XY UMa relative to SAO 27139 at
primary minimum (stars), secondary minimum (circles), maximum after
primary eclipse (squares), maximum after secondary eclipse
(triangles). The solid line is the mean of the two maxima.
\large
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\noindent
relatively symmetric primary minima. Thus our eclipse timings {\it do
not} necessarily support the third body hypothesis for XY~UMa, although
a longer period might be indicated.

The appearance of the light curve has changed markedly between
1993 and 1995, with the secondary minimum
becoming more prominent with respect to primary minimum. The scatter
apparent in the 1993 light curve from phases 0.2 and 0.4 seems to be
genuine variation of the light curve, between data taken on different
nights (see also HB94).
We have parameterised the light curves by estimating the differential V
magnitudes of primary and secondary minima (Min$_{1}$ and Min$_{2}$)
and the maxima between
phases 0 and 0.5 (Max$_{1}$) and between phases 0.5 and 1.0 (Max$_{2}$).
These estimates should be accurate to $\pm0.02$ and are with respect to
BD\,+55\degr\,1320.

\begin{center}
\begin{tabular}{lcccc}
\hline
Year  & Min$_{1}$ & Min$_{2}$ & Max$_{1}$ & Max$_{2}$ \\
\hline
1993  & 0.46 & 0.03 & $-$0.15 & $-$0.14 \\
1995  & 0.41 & 0.18 & $-$0.01 & $-$0.09 \\
\hline 
\end{tabular}
\end{center}


To compare our results with
previous work, we standardise the differential magnitudes above to
what they would be had the more commonly used 
SAO\,27139 been the comparison star. This is
simply achieved by subtracting 0.07 magnitudes, because SAO\,27139 is
0.07 magnitudes fainter than BD\,+55\degr\,1320 (HB94). Data from our two
light curves is added to the data in HC95 along with a compilation of
differential magnitudes of maxima and minima taken
between 1955 and 1984, from Lee (1985). The full dataset is shown in
Figure~3.
As suggested by Lee (1985) and HC95 there is a definite
trend in the data indicating that the whole system has been getting
brighter, in a phase independent way,
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\centerline{4}
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\noindent since about 1961.
We interpret this in terms of the model developed by HC95,
where long term system brightness changes over decades, are likely to
be caused by changes in the area covered by both high and low latitude 
spots, visible both
in and out of eclipse. Short term changes from year to year are more
likely caused by changes in the coverage and axial symmetry of low
latitude spots which may be obscured during primary eclipse, as the
system inclination is 82\degr (HB94). If a magnetic cycle, similar
to the Sun's exists, then it appears to be at least 40 years in length.

HC95 also suggest that, having gone through a period of low spot
activity in the last ten years, XY UMa may now be commencing a period
of increasing spot activity. Some interesting, but inconclusive
evidence for this comes from our 1995 light curve. Figure~3 shows that
while the primary minimum brightness is still increasing, the hemisphere
facing away from the secondary appears to be growing darker. Such
anti-correlations do not seem common in Figure~3. A possible
interpretation is that while the area covered by high latitude spots
is still decreasing, and thus the brightness at primary eclipse
increasing, there has been an eruption of spots at lower
latitudes, causing a decrease in system brightness at phases other than
primary eclipse. It maybe that these will then migrate towards higher
latitudes, presaging a new cycle of overall decreasing system
brightness and increasing spot activity. An
alternative explanation would be that the low latitude spots have
simply become highly asymmetric, favouring the hemisphere facing away
from the secondary. Further observations over the next couple of years
should provide some answers.

Historical brightness of XY UMa relative to SAO 27139 at
primary minimum (stars), secondary minimum (circles), maximum after
primary eclipse (squares), maximum after secondary eclipse
(triangles). The solid line is the mean of the two maxima.
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\beginauthors
R. D. JEFFRIES 
Department of Physics
Keele University
Keele, Staffordshire ST5 5BG, UK.
e-mail rdj@astro.keele.ac.uk
\vskip 14mm
\authorsrightcol
C. COLLINS, 
K. H. ELLIOTT,
S.~F.~HELSDON,
J. M. PITTARD 
S.~B.~TAYLOR
School of Physics and Space Research
University of Birmingham
Birmingham B15 2TT, UK
\endauthors

\references

Ar\'{e}valo, M. J., L\'{a}zaro, C., Martinez-Roger, C., Fuensalida,
J. J., 1994, {\it ASP Conference Series},
{\bf 64}, 548, in {\it 8th Cambridge Workshop on Cool Stars Stellar
Systems and the Sun}, ed. J. P. Caillault
%Changed: Wed Jun  1 11:33:48 MEST 2005

Bedford, D. K., Jeffries, R. D., Geyer, E. H., Vilhu, O., 1990, {\it
Mon. Not. R. astr. Soc.}, {\bf 243}, 557

Hilditch, R. W., Collier Cameron, A., 1995, {\it
Mon. Not. R. astr. Soc.}, {\bf 277}, 747

Hilditch, R. W., Bell, S. A., 1994, {\it Mon. Not. R. astr. Soc.}, {\bf
267}, 1081

Lee, W-B., 1985, {\em PhD thesis}, Universit\"{a}t Bonn

Pojma\'{n}ski, G., Geyer, E. H., 1990, {\it Acta. Astron.}, {\bf 40},
245


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