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\begin{document}
\IBVShead{5679}{20 January 2006}

\IBVStitle{New Eclipsing Variables in the Field of M67}


\IBVSauth{SANDQUIST, E. L.}

\IBVSinsto{Department of Astronomy, San Diego State University, 5500 Campanile
Drive, San Diego, CA 92182 USA; e-mail: erics@sciences.sdsu.edu}

\SIMBADobjAlias{Cl* NGC 2682 FBC 2404}{Cl* NGC 2682 MMJ 5151}
\SIMBADobjAlias{Cl* NGC 2682 FBC 5018}{Cl* NGC 2682 SAND 1601}
\SIMBADobj{Cl* NGC 2682 FBC 5774}
\SIMBADobj{Cl* NGC 2682 FBC 5986}
\IBVStyp{ E }
\IBVSkey{photometry}

\IBVSabs{We present four newly-discovered eclipsing systems}
\IBVSabs{(1 detached, 3 W UMa systems) in the field of the open cluster M67.}
\IBVSabs{Only two of the variables (the detached system and one W UMa)}
\IBVSabs{might be cluster members, but low proper motion probabilities or}
\IBVSabs{unusual color-magnitude diagram positions complicate the situation.}


\begintext

\vspace*{-0.8cm}

The old open cluster M67 has been extensively studied photometrically, but its
variable star content has not been completely surveyed due in part to its
large angular diameter. We have discovered 4 new eclipsing variable stars in
our photometric data for the cluster taken using the 1 m telescope at Mount
Laguna Observatory. The newly discovered variables are listed in Table 1. The
identification numbers are from Fan et al. (1996; abbreviated FBC) and Sanders
(1977). The photometry and period estimate comes from the current study (with
the tabulated magnitudes being estimates of maximum light).  Proper motion
membership probabilities come from Sanders (1977; labelled S), Girard et al.
(1989; labelled G), and Zhao et al. (1993; labelled Z). The color-magnitude
diagram (CMD) positions for the stars are shown in Figure 1.

\IBVSfig{9cm}{5679-f1.eps}{$VI$ color-magnitude diagram for M67 stars,
  with the identified variables indicated.}
\IBVSfigKey{5679-f1.eps}{M67}{colour-magnitude diagram}

\begin{table}[!ht]
\normalsize
\centerline{Table 1. New Eclipsing Variables in the Field of M67.}
\vskip 3mm
\begin{center}
\begin{tabular}{cccccccccc}
\hline
FBC ID   &  S ID  &  max($V$) & max($I$) & $V$ amp. & $I$ amp. & $P$ (d) &
\multicolumn{2}{c}{$P_\mu$} & Type \\
\hline
2404 &  743 & 15.66 & 14.74 & 0.10 & & & 16 (S) & & EA \\%partial eclipse
5018 & 1601 & 14.32 & 13.33 & 0.13 & 0.11 & 0.54 & 91 (S) & 48 (G) & EW\\
5774 & & 17.94 & 16.88 & 0.31 & 0.30 & 0.28 & & & EW\\
5986 & 1849 & 13.78 & 13.12 & 0.15 & 0.12 & 0.445 & 0 (S) & 0 (Z) & EW\\
\hline
\end{tabular}
\end{center}
\vspace*{-0.6cm}
\end{table}

\vspace*{-0.8cm}

Two of the new systems have positions in the CMD near the main sequence for
M67 members, but have small ($< 20\%$) proper motion membership probabilities.
Because M67 has a relatively small distance from the Sun, a velocity of 1 km
s$^{-1}$ relative to the cluster motion in the plane of the sky would produce
an apparent proper motion of 0.25~mas y$^{-1}$. Dynamical interactions
between stars appear to play a significant role in M67 thanks to a high binary
star content and low velocity dispersion (e.g. Sandquist 2005), so we need to
bear in mind that a relatively small kick velocity ($\sim 10$ km s$^{-1}$)
resulting from a 3- or 4-body interaction could give a cluster member a
membership probability of less than 20\% in proper motion studies. The
possibility of kicks can be examined by looking at the magnitude of the
measured proper motions (which are generally measured relative to
high-probability cluster members) and the position of the star in the vector
point diagram. If a star falls near the center of the field star proper motion
distribution, a kick would be a less likely explanation.

The calibrated $VI$ photometric data for the variables are provided in Tables
2 - 5, available on the IBVS website as {\tt 5679-t2.txt - 5679-t5.txt} The columns are heliocentric Julian date $-$ 2450000.0, magnitude,
magnitude error, and filter band.


{\bf FBC 2404 (S743):} The one proper motion study (Sanders 1977) that covered
this detached eclipsing system gave it a low, but nonzero, membership
probability.  Its position close to the cluster main sequence in the CMD,
however, hints that the binary might be a cluster member. The system was just
outside of the Chandra field observed by van den Berg et al. (2004), and it
was not detected by Belloni et al. (1998). The nondetection would not be
surprising because of the faintness of the system, especially if the orbital
period is more than a few days.

We observed a single partial eclipse, and see no sign of significant
variation outside of eclipse. The eclipse is depicted in Figure 2.

\IBVSfig{8cm}{5679-f2.eps}{Eclipse observed for the star FBC 2404.}
\IBVSfigKey{5679-f2.eps}{FBC 2404}{light curve}

\vskip 0.3cm

{\bf FBC 5018 (S1601):} The proper motion membership probabilities for this
system identify it as a possible cluster member. Girard et al. (1989) measured
a larger proper motion (3.2 mas y$^{-1}$), which is responsible for their
lower membership probability. The color-magnitude diagram position is unusual,
however: the system is bluer than the locus for equal-mass binaries.  If the
system is truly part of the cluster, there must be at least one other star
contributing significantly to the system light. If it is not a cluster member,
it is unusually bright for its properties: the $M_I$-period-color relation
(Rucinski 1997) for W UMa binaries returns a distance modulus that is about
0.7 mag {\it larger} than those of cluster member systems in contradiction to
its position brighter than M67's main sequence. van den Berg et al.  (2004)
identified this star in Chandra X-ray observations (CX7), and they found a
count rate that was more than twice as large as any of the previously known
cluster W UMa variables. The contradictions among the methods we have used to
check on membership leads us to recommend that this system be studied further
to clarify its unusual nature.

Although we have not observed the binary through an entire orbital cycle, we
did observe two maxima in $I$-band, allowing us to estimate the period (see
Figure 3).  The first maximum we observed appeared to be slightly fainter than
the second, which could indicate the presence of starspots.

\IBVSfig{9cm}{5679-f3.eps}{Observations for the W UMa-type variable star
  FBC 5018.}
\IBVSfigKey{5679-f3.eps}{FBC 5018}{light curve}

\vspace*{-0.5cm}

{\bf FBC 5774:} No proper motion information is available, but the system
falls far to the red of the cluster main sequence, so that it is very unlikely
to be a member. Use of the $M_I$-period-color relation (Rucinski 1997)
indicates that the system has a distance modulus more than 2.5 magnitudes
larger than the 4 previously known cluster W UMa stars. The amplitude of
the variable is fairly large ($\sim 0.3$ mag), and one of the photometric
minima is definitely deeper than the other (See Figure 4).

\IBVS2fig{7cm}{5679-f4.eps}{Observations for the W UMa-type variable star FBC 5774.}{5679-f5.eps}{Observations for the W UMa-type variable star FBC 5986.}
\IBVSfigKey{5679-f5.eps}{FBC 5986}{light curve}
\IBVSfigKey{5679-f4.eps}{FBC 5774}{light curve}

\newpage

{\bf FBC 5986 (S1849):} This contact system happens to lie very close to the
cluster main sequence in the CMD. However, two proper motion studies give the
system a 0\% probability of membership, thanks to measured relative proper
motions of 6.8 (Sanders 1977) and 9.1 mas y$^{-1}$ (Zhao et al. 1993). Based
on this, the system is unlikely even to be a member in the process of
ejection. Use of the $M_I$-period-color relation also indicates that its
distance modulus is about 0.8 mag larger than the cluster W UMa stars. This
system was not in the field observed in X-rays by van den Berg et al. (2004)
or Belloni et al. (1998). The light curve is fairly symmetrical (see Figure
5).


\vskip 0.5cm

This work has been funded through grants AST 00-98696 and 05-07785 from
the National Science Foundation to E.L.S. and M. Bolte.

\vskip 2mm

\references

Belloni, T., Verbunt, F., \& Mathieu, R.D. 1998, {\it A\&A}, {\bf 339}, 431. 
%\BIBCODE{1998A&A...339..431B}

Fan, X., et al. 1996, {\it AJ}, {\bf 112}, 628. 
%\BIBCODE{1996AJ....112..628F}

Girard, T.M., Grundy, W.M., Lopez, C.E., \& van Altena, W.F. 1989, {\it AJ},
{\bf 98}, 227. 
%\BIBCODE{1989AJ.....98..227G}

Rucinski, S.M. 1997, {\it AJ}, {\bf 113}, 407. 
%\BIBCODE{1997AJ....113..407R} 

Sanders, W.L. 1977, {\it A\&AS}, {\bf 27}, 89. 
%\BIBCODE{1977A&AS...27...89S}

Sandquist, E.L. 2005, {\it ApJL}, {\bf 635}, L73. 
%\BIBCODE{2005ApJ...635L..73S}

van den Berg, M., Tagliaferri, G., Belloni, T., \& Verbunt, F. 2004, {\it
  A\&A}, {\bf 418}, 509. 
%\BIBCODE{2004A&A...418..509V}

Zhao, J.L., Tian, K.P., Pan, R.S., He, Y.P., \& Shi, H.M. 1993, {\it A\&AS},
{\bf 100}, 243. 
%\BIBCODE{1993A&AS..100..243Z}

\endreferences

\IBVSedata{5679-t2.txt}
\IBVSedata{5679-t3.txt}
\IBVSedata{5679-t4.txt}
\IBVSedata{5679-t5.txt}

\end{document}

