\documentstyle[twoside, epsfig]{article}

\input ibvs2.sty

\begin{document}

\IBVShead{5685}{8 February 2006}

\IBVStitletl{Z G\lowercase{ru} and GSC 9092-1397 are Double-Mode}{RR Lyrae Variable Stars}

\IBVSauth{Wils, Patrick}
\IBVSinsto{Vereniging Voor Sterrenkunde, Belgium; e-mail: patrick.wils@cronos.be}

\SIMBADobj{GSC 9092-1397}
\SIMBADobj{Z Gru}
\GCVSobj{Z Gru}

\IBVStyp{RRd, RR(B)}
\IBVSkey{Variable stars, RR Lyrae stars, Double-mode variability}
\IBVSabs{The RR Lyrae stars Z Gru and GSC 9092-1397 are identified as}
\IBVSabs{double-mode RR Lyrae stars from the publicly available ASAS-3 data.}


\begintext

Recently a number of Galactic field double-mode RR Lyrae stars (type RRd) have been discovered
in the publicly available data from the Northern Sky Variability Survey ({\it NSVS}; Wozniak et al., 2004)
and the All Sky Automated Survey ({\it ASAS-3}; Pojmanski, 2002).
The most recent discoveries were announced by Oaster, Smith \& Kinemuchi (2005) and Wils, Lloyd \& Bernhard (2006).
Still, the number of field RRd stars is much lower than for example in the Large Magellanic Cloud (Alcock et al., 2000).

Further examination of the southern RR Lyrae stars listed by 
the {\it ASAS-3} survey, has revealed two more previously unknown double-mode RR Lyrae stars: Z~Gru 
and GSC~9092-1397.

Table 1 lists fundamental light curve parameters
for these stars, derived from the {\it ASAS-3} data.  
It includes values for the invariant Fourier parameters and 
for the generalized phase differences $G_{1,1}$ and $G_{-1,1}$
of the cross coupling terms $f_0 + f_1$ and $f_1 - f_0$ respectively 
as defined by Poretti and Pardo (1997).  
Formal uncertainties are given between parentheses in units of the last significant decimal.
Also listed are the Galactic latitude $b$ in degrees, the position and
the total proper motion $\mu$ derived from the {\it UCAC2} catalogue (Zacharias et al., 2004), 
and the {\it 2MASS} (Cutri et al., 2003) colour index. 
The electronic version of the IBVS contains direct links to the {\it ASAS-3} source data. 

\begin{table}
\begin{center}
\caption{Characteristics of the two new double-mode RR Lyrae stars} \vskip2mm
\label{new}
\begin{tabular}{ccc} 
\hline
Star                 & GSC~9092-1397 & Z~Gru \\
\hline
$V_{ASAS-3}$          & 12.91-13.71   & 12.01-12.70 \\
HJD Maximum          & 2452535.59    & 2452909.70 \\
Period F (d)         & 0.49152(5)    & 0.48800(5) \\
Period 1O (d)        & 0.36574(3)    & 0.36319(3) \\
Period ratio         & 0.7441(1)     & 0.7442(1) \\
$R_{21}$(F)          & 0.23(5)       & 0.26(3) \\
$R_{21}$(1O)         & 0.21(4)       & 0.17(2) \\
$\Phi_{21}$(F)       & 3.92(9)       & 3.65(10) \\
$\Phi_{21}$(1O)      & 4.84(19)      & 4.99(10) \\
Amplitude ratio 1O/F & 1.17(7)       & 1.30(5) \\
$G_{1,1}$            & 3.97(11)      & 3.92(8) \\
$G_{-1,1}$           & 3.56(18)      & 3.82(8) \\
$b$                  & $-$29.5       & $-$46.5 \\
RA (2000)            & $19\hr39\mm33\fsec50$ & $21\hr34\mm37\fsec12$ \\
Dec (2000)           & $-65\deg28\arcm51\farcs1$ & $-49\deg07\arcm28\farcs6$ \\
$\mu$ (mas/yr)       & 22.3          & 33.6 \\
$J-K_s$              & 0.28          & 0.29 \\
\IBVSshortlink{http://www.astrouw.edu.pl/\~gp/asas/asas.html}{ASAS-3} &
\IBVSshortlink{http://www.astrouw.edu.pl/cgi-asas/asas_lc/193933-6528.9}{193933-6528.9} &
\IBVSshortlink{http://www.astrouw.edu.pl/cgi-asas/asas_lc/213437-4907.5}{213437-4907.5} \\
\hline
\end{tabular}
\end{center}
\end{table}

The plots in Figs.~1 to 4 give the phase diagrams for both stars for the fundamental mode and the first overtone mode,
in both cases prewhitened for the other mode and its harmonics.

\IBVS2fig{6cm}{5685-f1.eps}{{\it ASAS-3} phased light curve for the fundamental period of Z~Gru.}{5685-f2.eps}{{\it ASAS-3} phased light curve for the first overtone period of Z Gru.}
\IBVSfigKey{5685-f1.eps}{Z Gru}{light curve}
\IBVSfigKey{5685-f2.eps}{Z Gru}{light curve}

\IBVS2fig{6cm}{5685-f3.eps}{{\it ASAS-3} phased light curve for the fundamental period of GSC~9092-1397.}{5685-f4.eps}{{\it ASAS-3} phased light curve for the first overtone period of GSC 9092-1397.}
\IBVSfigKey{5685-f3.eps}{GSC 9092-1397}{light curve}
\IBVSfigKey{5685-f4.eps}{GSC 9092-1397}{light curve}

Table 2 lists all isolated Galactic field double-mode RR Lyrae stars known in the literature, in the order of increasing
fundamental frequency.  The table also gives the first overtone frequency and the frequency and amplitude ratio. 
Missing values in the literature were derived from {\it NSVS} or {\it ASAS-3} data when possible and are marked with an asterisk.
This list does not include the double-mode RR Lyrae stars found in the Galactic Bulge (Moskalik \& Poretti, 2003; Pigulski et al., 2003; Mizerski, 2003), 
nor the Galactic foreground stars to the Sagittarius dwarf galaxy (Cseresnjes, 2001). \\
The double-mode nature of BS~Com was suggested by Bragaglia et al. (2003), but {\it ASAS-3} and {\it NSVS} data
demonstrate its reality.


\begin{table}
\begin{center}
\caption{Galactic field double-mode RR Lyrae stars} \vskip2mm
\label{all}
\begin{tabular}{lllllr} 
\hline
Star          & $f_0$   & $f_1$ & Ratio & $A_1/A_0$ & Ref. \\
\hline
GSC 4868-0831 & 1.7733  & 2.3764  & 0.7462  & 2.45  & 1 \\
AQ Leo        & 1.8182  & 2.4369  & 0.746   & 1.65* & 2 \\
CU Com        & 1.8377  & 2.4645  & 0.7457  & 2     & 3 \\
GSC 4421-1234 & 1.8491  & 2.4802  & 0.7456  & 2.25* & 4 \\
GSC 8936-2145 & 1.9335  & 2.5960  & 0.7448  & 1.37  & 1 \\
EN Dra        & 1.9537  & 2.6228  & 0.7449  & 2.03  & 5 \\
GSC 3059-0636 & 2.0243  & 2.7255  & 0.7427  & 0.38  & 6 \\
GSC 9092-1397 & 2.0345  & 2.7342  & 0.7441  & 1.17  & 7 \\
BS Com        & 2.0463* & 2.7544* & 0.7429* & 1.42* & 8 \\
Z Gru         & 2.0492  & 2.7534  & 0.7442  & 1.30  & 7 \\
V458 Her      & 2.0673  & 2.7780  & 0.7442  & 2.17* & 4 \\
GSC 3047-0176 & 2.1070  & 2.8330  & 0.7437  & 1.29* & 9 \\
V372 Ser      & 2.1220  & 2.8507  & 0.7444  & 1.4   & 10 \\
GSC 8403-0647 & 2.1376  & 2.8754  & 0.7434  & 1.05  & 1 \\
EM Dra        & 2.1518  & 2.8953  & 0.7432  & 1.18  & 5 \\
V2493 Oph     & 2.1582  & 2.9050  & 0.7429  & 1.58  & 11 \\
GSC 7411-1269 & 2.1680  & 2.9199  & 0.7425  & 0.98  & 1 \\
\hline
\end{tabular}
 \medskip
 \begin{tabular}{rl}
\multicolumn{2}{l}{References:}\\
$^1$ & Wils \& Otero, 2005 \\
$^2$ & Jerzykiewicz \& Wenzel, 1977 \\
$^3$ & Clementini et al., 2000 \\
$^4$ & Wils, Lloyd \& Bernhard, 2006 \\
$^5$ & Clement, Kinman \& Suntzeff, 1991 \\
$^6$ & Oaster, Smith \& Kinemuchi, 2005 \\
$^7$ & This paper \\
$^8$ & Bragaglia et al., 2003 \\
$^9$ & Koppelman et al., 2004 \\
$^{10}$ & Garc\'{\i}a-Melendo, Henden \& Gomez-Forrellad, 2001 \\
$^{11}$ & Garc\'{\i}a-Melendo \& Clement, 1997 \\
\end{tabular}
\end{center}
\end{table}

\newpage

{\bf Acknowledgements:} 
Ennio Poretti and John Greaves are acknowledged for helpful comments.
This research has utilised the {\it ASAS-3} public photometry catalogue and the SIMBAD and VizieR 
databases operated at the {\it Centre de Donn\'ees Astronomiques (Strasbourg)} in France.
Use was made of the data products from the {\it Two Micron All Sky Survey}, 
which is a joint project of the University of Massachusetts 
and the Infrared Processing and Analysis Center/California Institute of Technology, 
funded by the National Aeronautics and Space Administration 
and the National Science Foundation.


\references

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\endreferences

\end{document}

