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\IBVShead{5744}{2 January 2007}

\IBVStitletl{Newly Discovered Variable Stars}{ in the Globular Cluster NGC 1261}

\IBVSauth{Salinas, R.$^{1,2}$; Catelan, M.$^2$; Smith, H.A.$^3$; Pritzl, B.J.$^4$}

\IBVSinst{Grupo de Astronom\'{\i}a, Facultad de Ciencias F\'{\i}sicas y Matem\'aticas,
Universidad de Concepci\'on, Concepci\'on, Chile; email: rsalinas@astro-udec.cl}

\IBVSinst{Pontificia Universidad Cat\'olica de Chile, Departamento de Astronom\'{\i}a y
Astrof\'{\i}sica, Av.\ Vicu\~na Mackenna 4860, 782-0436 Macul, Santiago, Chile; 
email: mcatelan@astro.puc.cl}

\IBVSinst{Department of Physics and Astronomy, Michigan State University, 
East Lansing, MI 48824, USA; \\ email: smith@pa.msu.edu}

\IBVSinst{Macalester College, 1600 Grand Avenue, Saint Paul, MN 55105, USA; 
email:  pritzl@macalester.edu}


\SIMBADobj{NGC 1261}
\IBVSabs{We report the discovery of several RR Lyrae and SX Phe variable stars}
\IBVSabs{in the innermost part of the globular cluster NGC 1261. Preliminary}
\IBVSabs{pulsation periods, location and B-band light curves in relative flux}
\IBVSabs{units are provided for all these stars.}

\begintext

NGC~1261 (RA 03\hr12\mm15\fsec3, DEC $-55$\deg13\arcm01\arcs, J2000) is a distant
($R_{GC}=18.2$~kpc; Harris, 1996) globular cluster with an intermediate metallicity (${\rm
[Fe/H]}=-1.35$) and horizontal branch (HB) morphology not unlike NGC~1851's, with evidence of
an HB bimodality --- i.e., with fewer known RR Lyrae variables than either red HB or blue HB
stars (Ferraro et al., 1993).

The RR Lyrae population in the cluster was discovered in photographic studies by Laborde \&
Fourcade (1966), Bartolini et al.\ (1971), Wehlau \& Demers (1977), and Wehlau
et al.\ (1977). To the best of our knowledge, no CCD study tackling the variable star
populations in this cluster has ever appeared in the literature. In the present note, we
report on the discovery of additional variable stars in the cluster.  This work is part of a
larger effort to bring to light the variable star population properties of several globular
clusters that have not been properly investigated with modern CCD images (Catelan et al.,
2006).
 
The cluster images were obtained using the Danish 1.54-m telescope at La
Silla, Chile, from September 17 to September 22, 2005. The $2048 \times
2048$ RINGO CCD was used. Given its 0\farcs395 pixels, the total observed
field was $13\farcm 5 \times 13\farcm 5$. The time series data consist 
104 $B,V$ pairs, with typical exposure times of 100~sec in $B$ and 35~sec
in $V$. Here we report the results based on the $B$ data only.

To search for variability in our data, we have adopted the image subtraction technique (ISIS
v2.1; Alard, 2000). In recent years, this technique has provided the most powerful tools for
finding variable stars in crowded regions without the need of large apertures (e.g., Olech et
al., 1999; Contreras et al., 2005). Its main drawback is the difficulty to reliably transform
relative fluxes into calibrated magnitudes, and even to derive accurate pulsation amplitudes
(Corwin et al., 2006, and references therein).

Making use of ISIS we were able to re-discover 19 out of the 21 variables listed in the
Clement et al.\ (2001) catalog, confirming the non-variability of V1 already noted by Wehlau
\& Demers (1977), but not finding any variability for V18.  The latter appears rather
surprising, given that Wehlau et al.\ (1977) found a very precise period ($P=0.33653$~d) for
V18.  However, taking the original data for V18 from Table~1 in Wehlau et al.\ (1977), we do
not find any period that phases the data correctly (Fig.~1). Considering that the position of
this variable is only 25\arcs\ from the cluster center, and that the magnitudes of Wehlau et
al.\ (1977) were derived by eye, we are confident to discard it as an RR Lyrae star.  In the
case of V19, Wehlau et al.\ (1977) do not give a period; we estimate it to be near 0.653~d.  
For the rest of the known variables we agree with the periods listed in the Clement et al.\
(2001) catalog.

\IBVSfig{5.5in}{5744-f1.eps}{Light curve of V18, using data from Wehlau et al.\ (1977), with a
period of 0.33653~d}
\IBVSfigKey{5744-f1.eps}{V18 in NGC 1261}{light curve}

\IBVSfig{4.4in}{5744-f2.ps}{Finding chart for the innermost variable stars in NGC~1261. The
field size is approximately $3'\times 2'$. North is up and East to the left}
\IBVSfigKey{5744-f2.ps}{NGC 1261}{finding chart}

\IBVSfig{6.1in}{5744-f3.eps}{$B$-band differential light curves for previously known variable
stars in NGC~1261}
\IBVSfigKey{5744-f3.eps}{}{light curve}

\IBVSfig{6.1in}{5744-f4.eps}{$B$-band differential light curves for the known variables V19,
V20 and V21; and for all the newly identified variables. Note that the light curves of V15 and
V23 are not phased}
\IBVSfigKey{5744-f4.eps}{}{light curve}

\vspace{-36pt}

\begin{table}
\begin{center}
\caption{Locations and tentative periods for new variable stars in 
NGC~1261}
\smallskip
\begin{tabular}{lrrll}
\hline
Variable  & $x~(\arcs)$ & $y~(\arcs)$ & Period (d) & Type   \\ 
\hline
V22       & $4.1$    & $-41.3$ & 0.302    & RRc  \\ 
V23       & $-2.3$   & $15.9$  &  --      & LPV  \\  
V24       & $-13.1$  & $-37.8$ & 0.626    & RRab \\
V25       & $11.2$   & $94.3$  & 0.0535   & SX Phe\\
V26       & $9.5$    & $12.9$  & 0.0799   & SX Phe\\
V27       & $-11.6$  & $-9.5$  & 0.341    & RRc   \\
V28       & $-20.9$  & $-3.6$  & 0.287    & RRc \\
V29       & $-25.3$  & $-23.4$ & 0.593    & RRab \\ 
V30       & $4.9$    & $3.0$   & 0.0591   & SX Phe \\  
\hline
\end{tabular}
\end{center}
\end{table}

Also we have found nine new variables of different types: one long period variable (LPV),
three SX~Phoenicis and five RR~Lyrae stars (3 RRc and 2 RRab). The location, classification
and tentative periods for these new variables are given in Table~1. In this table, the $x$ and
$y$ coordinates are in arcseconds with respect to the cluster center, as given in the online
Clement et al.\ (2001) catalog. Also a finding chart with all the new variables can be seen in
Figure~2.

Due to the relatively small time coverage, it is not possible to give an estimate of the
period of V23. For the RR~Lyrae stars we think periods are good only up to the third decimal
place, and for the SX~Phe variables we can determine periods to four significant figures.

With our new discoveries, and assuming the new RR Lyrae stars to be cluster members, the value
of $\langle P_{ab}\rangle$ changes slightly with respect to Wehlau et al.\ (1977), from
0.555~d to 0.568~d, and $N_c/(N_{c}+N_{ab})$ changes from 0.26 to 0.30. In addition, one finds
$\langle P_{c}\rangle = 0.319$~d, $P_{ab}^{\rm min} = 0.49286$~d, and $P_{c}^{\rm max} =
0.341$~d.  These results do not change NGC~1261's classification as an Oosterhoff type I
cluster.

\medskip

{\bf Acknowledgements.} We thank R. Leiton for helping us with the data transfer from
Concepci\'on to Santiago. R.S. acknowledges support by a CONICYT Doctoral Fellowship.  M.C.
acknowledges support by Proyecto FONDECYT Regular No.~1030954. H.A.S.  acknowledges the NSF
for support under grant AST 02-05813.

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\endreferences

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