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\IBVShead{5533}{4 June 2004}
\IBVStitletl{Newly Discovered Variable Stars}{ in the Globular Cluster NGC~1851}

\IBVSauth{SUMEREL, A. N.$^1$; CORWIN, T. M.$^{1,4}$; CATELAN, M.$^{2,4}$;  BORISSOVA, J.$^{2,4}$; SMITH, H. A.$^3$}

\IBVSinst{Department of Physics, University of North Carolina at Charlotte, Charlotte, NC 28221, USA; \\   
       e-mail: ansumere, mcorwin@uncc.edu}
\IBVSinst{Pontificia Universidad Cat\'olica de Chile, Departamento de Astronom\'\i a y Astrof\'\i sica, Av
. Vicu\~{n}a Mackenna 4860, 782-0436 Macul, Santiago, Chile; email: mcatelan, jborisso@astro.puc.cl}
\IBVSinst{Department of Physics and Astronomy, Michigan State University, East Lansing, MI 48824, USA; \\  
 email: smith@pa.msu.edu}
\IBVSinst{Visiting Astronomer, European Southern Observatory, La Silla, Chile}

\SIMBADobj{NGC 1851}

\IBVSabs{We report on the discovery of 19 previously unknown variable stars}
\IBVSabs{in NGC~1851, most of which of the RR Lyrae type. Preliminary periods}
\IBVSabs{and light curves (in $B$-band relative fluxes) are provided for all}
\IBVSabs{the newly detected variables.}

\begintext

\vspace*{-1cm}

The Galactic globular cluster NGC~1851, which has recently been associated, along 
with NGC~2808, with a previously unknown dwarf galaxy in Canis Major (Martin et 
al. 2004), belongs (again like NGC~2808) to a relatively rare group of clusters 
that display bimodal horizontal-branch (HB) morphology (Catelan et al. 1998 and 
references therein). We have recently discovered a sizeable RR Lyrae population in
NGC~2808 (Corwin et al. 2004). Here we report on the discovery of 19 previously
unknown variables in NGC~1851. 

Prior to our study, thirty-three variables were 
known in NGC~1851, 29 of which of the RR Lyrae type. The RR Lyrae variables in 
NGC~1851 have most recently been studied by Walker (1998). The CCD images used 
in the present study were obtained with the Danish 1.54-m telescope located at 
the European Southern Observatory at La Silla, Chile. The instrument used was 
the DFOSC with a field of view of 13\farcm7 $\times$ 13\farcm7. The data 
reported here are from four nights, 2002 December 11/12 and 12/13 amd 2003 
February 18/19 and 19/20. The seeing was around 1.0~arcsec or better. 
Image-subtraction (Alard \& Lupton 1998; Alard 2000) analysis results are 
reported here for 65 $B$ (December) and 83 $B$ (February) images.

The location 
and tentative period of the variables are given in Table 1.The $x$ and $y$ 
coordinates are given in arcseconds and with respect to the cluster center, 
given in the Clement et al. (2001) catalog as R.A.~05\hr12\fmm4 and 
Dec~$-$40\deg05\farcm0 (1950). Because the data are relatively limited, 
the periods are not definite. Light curves based on the periods given in 
Table~1 are shown in Figure~1. Because two different reference frames were 
used to determine the differential flux for December and for February, the 
light curve data for December were adjusted to match as closely as possible 
the light curve data for February. Thus the exact relationship between these 
data sets might not be as shown in Figure~1. When the new variable was found 
in both the December and the February data, the different symbols represent 
data from each of the two runs. NV6, NV7, NV9, NV18, and NV19 were found only 
in the February data and in those cases the different symbols represent data 
from the two consecutive nights. 

As for the previously known variables, it 
should be noted that the period 0.426653~d given in Walker (1998) for V31 does 
not phase our December/February data properly. Our data suggest that the 
period may be either approximately 0.0016~d shorter or 0.0011~d longer than 
Walker's period.

As can be seen from Table~1 and Figure~1, we have likely 
detected about 15 previously unknown RR Lyrae variables in NGC~1851. Together 
with the 29 variables of this type previously known, this raises the specific 
frequency of RR Lyrae variables in the cluster from $S_{\rm RR} = 13.5$ 
(Harris 1996) to $S_{\rm RR} \simeq 20.5$. 

\vskip 1cm

\centerline{Table 1. Locations and tentative periods for new variables.}
\vskip 3mm

\begin{center}
\begin{tabular}{lrrcc}
\hline
Variable  & $x('')$  & $y('')$  & Period (d) & Type \\ 
\hline
NV1  (V34)    &38.77 & $-10.87$ & 0.515 & RRab  \\ 
NV2  (V35) &22.18 & 4.31 & 0.321 & RRc \\  
NV3  (V36)   &14.07 & 12.49 & 0.318 & RRc  \\ 
NV4  (V37)   &12.26 & $-13.97$ & 0.350 & RRc \\  
NV5  (V38)  &3.52 & $-13.30$ & 0.75? & RRab? \\ 
NV6  (V39)  &3.27 & $-1.46$ & 0.573 & RRab \\  
NV7  (V40)  &0.59 & 12.32 & 0.503 & RRab? \\  
NV8  (V41)  & $-1.26$ & 5.89 & 0.400 & RRab? \\ 
NV9  (V42)  & $-1.46$ & $-3.10$ & 0.341 & RRc  \\  
NV10 (V43)  & $-2.14$ & 10.31 & 0.283 & RRc \\ 
NV11 (V44)  & $-2.73$ & 0.83 & 0.253 & RRc? \\ 
NV12 (V45)  & $-4.68$ & 17.77 & 0.389 & RRab \\  
NV13 (V46)  & $-7.06$ & $-0.80$ & 0.297 & RRc \\ 
NV14 (V47)  & $-14.92$ & 14.48 & 0.283 & RRc \\ 
NV15 (V48)  & $-21.22$ & $-6.12$ & 0.520 & RRab \\ 
NV16 (V49)  & $-23.30$ & $-1.80$ & 0.267 & RRc \\ 
NV17 (V50)  & $-38.58$ & $-28.44$ & 0.327 & RRc \\
NV18 (V51)  & $-41.86$ & 31.50 & 0.507 & RRab \\
NV19 (V52)  & $-44.82$ & 21.60 & 0.401 & ? \\
\hline
\end{tabular}
\end{center}
\vskip 0.5cm

\IBVSTack{We thank J. M. Fern\'andez for his assistance 
during some of the observing runs. M.C. acknowledges support by 
Proyecto FONDECYT Regular No.~1030954. H.A.S. acknowledges the NSF for 
support under grant AST 02-05813.}

\references
Alard, C., 2000, {\it A\&AS}, {\bf 144}, 363 

Alard, C., \& Lupton, R.H., 1998, {\it ApJ}, {\bf 503}, 325 

Catelan, M., Borissova, J., Sweigart, A.V., Spassova, N., 1998, {\it ApJ}, 
{\bf 494}, 265

Clement, C.M., et al., 2001, {\it AJ}, {\bf 122}, 2587

Corwin, T.M., Catelan, M., Borissova, J., Smith, H.A., 2004, {\it A\&A}, in press 
(astro-ph/0405004)

Harris, W.E., 1996, {\it AJ}, {\bf 112}, 1487 (Feb. 2003 update) 

Martin, N.F., Ibata, R.A., Bellazzini, M., Irwin, M.J., Lewis, G.F., Dehnen, W., 
2004, {\it MNRAS},   {\bf 348}, 12 

Walker, A.R., 1998, {\it AJ}, {\bf 116}, 220

\endreferences

\IBVSfig{17cm}{5533-f1.eps}{$B$-band light curves for newly 
detected variable stars in NGC~1851. ISIS relative fluxes are shown in all 
cases. The candidate period used to phase the light curves is given in each 
plot. Note that such periods are quite uncertain in some cases.}
\IBVSfigKey{5533-f1.eps}{NGC 1851}{light curve}



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