\documentstyle[twoside, epsf, graphics]{article}

\input ibvs2.sty

\begin{document}
\IBVShead{5673}{18 January 2006}

\IBVStitle{VZ G\lowercase{ru}: a Blazhko-Type RR L\lowercase{yr}, not a CV}

\IBVSauth{TAPPERT, C.$^1$; SCHMIDTOBREICK, L.$^2$; CATELAN, M.$^1$}

\IBVSinst{Departamento de Astronom\'{\i}a y Astrof\'{\i}sica,
Pontificia Universidad Cat\'olica, Av. Vicu\~{n}a Mackenna 4860,
782-0436 Macul, Santiago, Chile,
email: ctappert@astro.puc.cl}
\IBVSinst{European Southern Observatory, Alonso de C\'ordova 3107,
763-0581 Vitacura, Santiago, Chile \\}

\SIMBADobjJ2000{VZ Gru}{22 33 50.26 -50 12 24.0}

\IBVStyp{RRab}
\IBVSkey{photometry}

\IBVSabs{We confirm VZ Gru as a variable star of type RRab, and thus refute} 
\IBVSabs{the suggested classification as a candidate cataclysmic variable.}
\IBVSabs{We derive a most probable pulsation period of 0.508(4) d and detect}
\IBVSabs{the presence of a Blazhko modulation whose period has yet to be}
\IBVSabs{determined.}


\begintext 

VZ Gru was first classified as an irregular variable (Is) by Meinunger 
(1976). It was observed later again by Meinunger (1979) who maintained her
initial classification, but as a side remark noted that the observed variations
would also fit a RR Lyrae-type variability. With this information the star is 
also included in the actual General Catalogue of Variable Stars (Samus et al.\ 
2004). Subsequently, the star has received little attention, until Cieslinski 
et al.\ (1997, 1998) presented photometric and spectroscopic data, 
respectively. Their observations showed VZ Gru to be moderately faint 
($\langle V\rangle = 15.2$), slightly bluish ($\langle U\!-\!B\rangle = 0.16$, 
$\langle B\!-\!V\rangle = 0.30$), and of spectral type F4--F6. The star caught 
our attention since it is included in the Downes et al.\ (2001) catalogue as a 
candidate cataclysmic variable (CV).

We observed the star in 4 nights in 2002 as a back-up target at the 0.9 m
CTIO/SMARTS telescope using direct CCD imaging in the $R$ passband. See
Table \ref{obs_tab} for details.

The data were reduced in the usual way, using IRAF\footnote{IRAF is 
distributed by the National Optical Astronomy Observatories.} routines for bias 
subtraction and flat-fielding. Aperture photometry and light curves were
obtained for the target and a number of field stars with IRAF's DAOPHOT
package and the stand-alone daomatch and daomaster programs (Stetson 1992).
The differential light curve for the target was computed with respect to
5 suitable comparison stars. 


\begin{table}[!hb]
\begin{center}
\caption[]{Log of observations.\medskip}
\label{obs_tab}
\begin{tabular}{l l l l l}
\hline\noalign{\smallskip}
date & HJD & $n_{\rm data}$ & $t_{\rm exp}$ [s] & $\Delta t$ [h]\\
\hline\noalign{\smallskip}
2002-09-17 & 2\,452\,535 & 60 & 90 & 2.30 \\
2002-09-18 & 2\,452\,536 & 70 & 90 & 2.74 \\
2002-09-19 & 2\,452\,537 & 70 & 90 & 2.63 \\
2002-09-22 & 2\,452\,540 & 100 & 120 & 4.17 \\
\hline\noalign{\smallskip}
\end{tabular}
\end{center}
\end{table}


\IBVSfig{10cm}{5673-f1.ps}{Individual light curves from 2002-09-17/18/19/22 (from left to right).
Note that the photometric error bars are included but do not exceed the size
of the symbols.}
\IBVSfigKey{5673-f1.ps}{VZ Gru}{light curve}

From the resulting light curves (Fig. 1) it becomes immediately
clear that VZ Gru is not a CV, but a pulsating variable of type RRab, in good
agreement with the spectral type determined by Cieslinski et al.\ (1998). In 
our observations we have been lucky to record a minimum or maximum for all 
nights, so that a determination of the pulsation period should be possible. 
However, Fig. 1 also demonstrates the presence of a Blazhko 
(1907) effect that changes the maximum and minimum magnitude and the slope
between those two extrema in VZ Gru (Table \ref{ex_tab}).

\begin{table}[tb]
\begin{center}
\caption[]{Differential $R$ magnitudes for the observed maxima and minima.
The errors have been estimated with respect to the spread of data points
near the extrema.\medskip}
\label{ex_tab}
\begin{tabular}{l l l}
\hline\noalign{\smallskip}
date & max & min \\
\hline\noalign{\smallskip}
2002-09-17 & $-$0.391(10) & \\
2002-09-18 & $-$0.357(05) & 0.537(10) \\
2002-09-19 &              & 0.529(08) \\
2002-09-22 &              & 0.488(05) \\
\hline\noalign{\smallskip}
\end{tabular}
\end{center}
\end{table}

In order to search for the period we prepared the data set by correcting for
the differences in the extrema with respect to the data from 2002-09-18, which
includes both a maximum and a minimum. A periodogram was then computed
using the Scargle (1982) algorithm implemented in MIDAS. The resulting 
frequency spectrum shows a large number of aliases (Fig.\ \ref{sca_fig}).
However, taking into account the distinctive shape of the RRab-type light
curve together with the period distribution of these systems, we find that
the peak corresponding to $P = 0.511~{\rm d}~= 12.3~\rm{h}$ represents the
only viable choice.

In order to estimate the uncertainty of this period we folded the data set with
a number of nearby trial periods. The overall best visual result was achieved 
with $P = 0.508$ d, with a range of acceptable periods within $\pm 
0.004$ d (Fig. 3). Note that the Blazhko effect possibly 
introduces a systematic uncertainty that cannot be resolved in our data set due
to the low number of recorded extrema, and that can be suspected to be somewhat 
larger than the uncertainty derived by us. Further observations with a much 
longer time base of this field RRab star will be needed to disentangle the 
fundamental pulsation from the Blazhko contribution, and to provide a period 
for the latter.

\IBVSfig{10cm}{5673-f2.ps}{Scargle periodogram. The period $P = 0.511$ d is indicated.\label{sca_fig}}
\IBVSfigKey{5673-f2.eps}{VZ Gru}{periodogram}

\IBVSfig{10cm}{5673-f3.ps}{Light curve folded on $P = 0.508$ d. Different symbols indicate
different nights (see Fig. 1).}
\IBVSfigKey{5673-f3.eps}{VZ Gru}{light curve}


\bigskip\noindent
{\Large \bf Acknowledgements:}\medskip
CT and MC acknowledge financial support by FONDECYT grants 1051078 and
1030954, respectively. We are indebted to J. Rossa for looking up the Meinunger
(1979) reference. CT also would like to thank K. Kolenberg for enlightening 
discussions on this object and RR Lyr stars in general. 
This work has made intensive use of the SIMBAD database operated at CDS, 
Strasbourg, France, and also of the period-search program Peranso (courtesy 
T. Vanmunster).

\newpage

\references
Blazhko, S., 1907, Astron. Nachr., 175, 325

Cieslinski, D., Jablonski, F. J., Steiner, J. E., 1997, A\&AS, 124, 55
%\BIBCODE{1997A&AS..124...55C}

Cieslinski, D., Steiner, J. E., Jablonski, F. J., 1998, A\&AS, 131, 119
%\BIBCODE{1998A&AS..131..119C}

Downes, R. A., Webbink, R. F., Shara, M. M., Ritter, H., Kolb, U., Duerbeck, 
H. W., 2001, PASP, 113, 764
%\BIBCODE{2001PASP..113..764D} 

Meinunger, I., 1976, Mitt. Ver\"and. Sterne, 7, 188
%\BIBCODE{1976MitVS...7..188M}

Meinunger, I., 1979, Ver\"off. Sternwarte Sonneberg, 9, 105
%\BIBCODE{1979VeSon...9..105M}

Samus, N. N., Durlevich, O. V., et al., 2004, 
VizieR On-line Data Catalog: II/250
\ADSBIBCODE{2004yCat.2250....0S}

Scargle, J. D., 1982, ApJ, 263, 835
%\BIBCODE{1982ApJ...263..835S}

Stetson, P. B., 1992, {\it ASP Conf. Ser.}, {\bf 25}, 297, 
in: {\em Astronomical Data Analysis Software and Systems 
I}, ed. D.M. Worrall, C. Biemesderfer \& J. Barnes
%\ADSBIBCODE{1992ASPC...25..297S}

\end{document}

