\documentclass[twoside]{article}

\input ibvs2.sty
\input epsf.sty

\begin{document}

\IBVShead{5524}{16 April 2004}

\IBVStitletl{TU~UM\lowercase{i}: A Contact Binary in a Triple System}

\IBVSauth{Pych, W.$^1$; Rucinski, S. M.$^2$}

\IBVSinst{David Dunlap Observatory, University of Toronto, P.O.~Box 360, Richmond Hill, Ontario, Canada L4C 4Y6\\ and~ Copernicus Astronomical Center, Bartycka 18, 00--716 Warszawa, Poland; email: pych@camk.edu.pl}
\IBVSinst{David Dunlap Observatory, University of Toronto, P.O.~Box 360,
Richmond Hill, Ontario, Canada L4C 4Y6\\}

\SIMBADobj{TU UMi}
\IBVStyp{ EW }
\IBVSkey{CCD photometry}
\IBVSabs{We present 4 CCD times of minimum of the contact binary TU UMi.}
\IBVSabs{Linear ephemeris for the orbit is given.}
\IBVSabs{Spectra have revealed that it belongs to a triple system.}


\begintext

Photometric variability of the star BD+76~544 (HIP~73047,
$V\simeq8.8$~mag.) was discovered by the {\sc Hipparcos} mission.
In the Variability Annex of the {\sc Hipparcos} Catalogue (ESA, 1997),
the star has been assigned the name TU~UMi. The full, peak-to-peak
amplitude of the variability is about 0.06~mag. and quality of the
light-curve is rather poor. Consequently, TU~UMi could only
be classified as a periodic variable with the period 0.188546~days.
Duerbeck (1997) used the period-color relation for
contact binaries (Rucinski, 1993) to search for contact binary
candidates in the Variability Annex of the {\sc Hipparcos}
catalogue (ESA, 1997). TU~UMi was found to be one of them
and listed with the doubled period. Nevertheless the possibility
of it being a pulsating variable could not be ruled out. In fact,
the star has been included in the catalogue of 
$\delta$~Sct stars (Rodr{\'\i}guez et al., 2000).

In the nights 2003 March 26/27 UT, April 09/10 UT and April 11/12 UT,
TU~UMi was observed spectroscopically at the David Dunlap Observatory,
University of Toronto. The obtained spectra have been analyzed using
a broadening-function method (Rucinski, 2002).
Figure~1 presents a map of broadening-functions plotted against corresponding
orbital phases. The most pronounced feature in this plot has a constant
radial velocity of about $+30$~km~s$^{-1}$. It corresponds to the third
body in the system. The peak of the signature of the primary (more massive)
component in the close binary is roughly five times lower than the
peak corresponding to the third body. This broad feature is best visible
at the phases around $0.25$ at radial velocities in the range from
$-50$~km~s$^{-1}$ to $+150$~km~s$^{-1}$. The peak of the signature of the
secondary component of the close binary is over ten times lower than the
peak corresponding to the third body and is barely detectable in the figure.
This feature is visible at phases around $0.25$ with radial velocities
about $-150$~km~s$^{-1}$ and at phases around $0.75$ with velocities
about $+200$~km~s$^{-1}$. The plot clearly shows that TU~UMi
is a triple system containing a close binary. The small amplitude of the
photometric variability is explained, at least in part, 
by the fact that about 55\% of the total light comes 
from the companion star ($L_3/(L_1+L_2) = 1.25 \pm 0.15$).
This causes difficulties in the analysis of
the spectroscopic data with large uncertainties in the
$K_i$ amplitudes. The results will be published in the 10-th paper
of the DDO spectroscopic radial-velocity series (Rucinski et al., 2004). 

\IBVSfig{10cm}{5524-f1.eps}{Broadening-functions}
\IBVSfigKey{5524-f1.eps}{TU UMi}{Broadening-functions}

Simultaneously with the spectroscopic observations, we conducted
unfiltered photometry of TU~UMi. We were using a 15 cm refractor finder
of the main 1.88m telescope, 
equipped with SBIG ST-6 CCD camera. The data have been reduced
using procedures within IRAF\footnote{IRAF is distributed by the
National Optical Astronomical Observatories, operated by the Association
of Universities for Research in Astronomy, Inc., under contact with the
National Science Foundation} package. The aperture photometry has been
done using DAOPhotII package (Stetson, 1987). Figure~2 presents the finding
chart from our observations. The size of the field is about
18$\times$15~arc minutes. The sum of the light from stars
marked as C1 and C2 has been used as the comparison for differential
photometry. Phased light-curve is presented in Figure~3. The times of the
minimum were calculated using Kwee and van Woerden (1956) method and
are listed in Table~I. A linear least squares fit to our times of
minimum yields the following linear ephemeris:
$${\rm HJD\ MIN} = 2452725.6262(64) + 0.37730(19)\times E$$
Since the temporal spacing between time of the minimum from the {\sc Hipparcos}
Catalogue (HJD=2448500.0690) and our observations is longer than 10,000
orbits, we are not able to predict this number with a precision better
than 2 orbits. Thus we cannot calculate the mean orbital period
based on both sets of photometric data.

\IBVSfig{8cm}{5524-f2.eps}{Finding chart}
\IBVSfigKey{5524-f2.eps}{TU UMi}{Finding chart}

\IBVSfig{10cm}{5524-f3.eps}{Light-curve}
\IBVSfigKey{5524-f3.eps}{TU UMi}{Light curve}

\IBVSedata{5524-t2.txt}
\IBVSdataKey{5524-t2.txt}{TU UMi}{photometry}

\begin{table}
\begin{center}
\large
\textbf {Table I}. Times of Minimum of TU~UMi, 2003 \vskip2mm
\begin{tabular}{ccccc}
\hline
Type & HJD (2450000.+) & Error & Epoch & O-C \\
\hline
Primary   & 2725.62604 & 0.00035 &  0.0 & -0.000166 \\
Primary   & 2739.59307 & 0.00036 & 37.0 &  0.006779 \\
Secondary & 2739.76890 & 0.00076 & 37.5 & -0.006041 \\
Secondary & 2741.66087 & 0.00029 & 42.5 & -0.000569 \\
\hline
\end{tabular}
\end{center}
\end{table}

We would like to thank H. DeBond and J. Thomson, the telescope operators
at David Dunlap Observatory for their cooperation during observations.
We also thank J. Thomson for help in the preparation of the article.
This study has been done while W. Pych held the NATO
Post-Doctoral Fellowship administered by the
Natural Sciences and Engineering Council of Canada (NSERC).
The research was also supported by the NSERC grant to S. M. Rucinski.

\references

Duerbeck, H. W., 1997, {\it IBVS}, 4513

ESA, 1997, {\it The {\sc Hipparcos} Catalogue}, ESA SP-1200

Kwee K. K., van Woerden, H., 1956, {\it BAN}, {\bf 12}, 327

Rodr\'{\i}guez, E., L{\'o}pez-Gonz{\'a}lez, M. J., L{\'o}pez de Coca, P.
2000, {\it A\&AS}, {\bf 144}, 469

Rucinski, S. M.,\ 1993, {\it in The Realm of Interacting Binary Stars}, eds.\
J.\ Sahade, G.E.\ McCluskey, Jr.\ and Y.\ Kondo, Dordrecht: Kluwer, {\bf p.\ 111}

Rucinski, S. M., 2002, {\it AJ}, {\bf 124}, 1746

Rucinski, S. M., et al., 2004, in preparation.

Stetson, P. B., 1987, {\it PASP}, {\bf 99}, 191


\end{document}

