\documentstyle[twoside, epsfig]{article}

\input ibvs2.sty

\begin{document}

\IBVShead{5593}{15 January 2005}

\IBVStitle{Four New Southern Double-Mode RR Lyrae Stars}

\IBVSauth{Wils, Patrick$^1$; Otero, Sebasti\'an A.$^2$}
\IBVSinst{Vereniging Voor Sterrenkunde, Belgium; e-mail: patrick.wils@cronos.be}
\IBVSinst{Grupo Wezen 1 88, Centro de Estudios Astron\'omicos (CEA); e-mail: varsao@fullzero.com.ar}

\SIMBADobj{GSC 4868-0831}
\SIMBADobj{GSC 7411-1269}
\SIMBADobj{GSC 8403-0647}
\SIMBADobj{GSC 8936-2145}

\IBVStyp{RRd, RR(B)}
\IBVSkey{Variable stars, RR Lyrae Stars, Double-mode variability}
\IBVSabs{Analysis of the publicly available ASAS3 data has lead to the}
\IBVSabs{discovery of four new Galactic double-mode RR Lyrae stars: }
\IBVSabs{GSC 4868-0831, GSC 7411-1269, GSC 8403-0647 and GSC 8936-2145.}
\IBVSabs{GSC 4868-0831 is the brightest RRd star known thus far.}

\begintext

Double-mode pulsators, and double-mode RR Lyrae stars (type RRd) in particular,
are very important in the study of pulsation models.  Although they are abundant
in the Magellanic Clouds (Alcock et al., 2000) and Galactic globular clusters (e.g. Walker and Nemec, 1996),
very few Galactic Field RRd stars are known.
The online edition of the General Catalogue of Variable Stars (GCVS, Kholopov et.al., 2003)
only lists five RRd stars (new GCVS type RR(B)).  
A few other field and faint Galactic Bulge RRd stars have been discovered since,  
but there is still an apparent lack of relatively bright RRd stars in the Southern hemisphere
and with negative Galactic latitudes.

By examining the publicly available data for the Southern RR Lyrae stars discovered by 
the {\it ASAS3} survey (Pojmanski, 2002), we found four previously unknown double-mode RR Lyrae stars.

Table~1 lists fundamental light curve parameters
for the four stars, derived from the {\it ASAS3} 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 errors are given between parentheses in units of the last significant decimal.
Also listed are the Galactic latitude $b$ in degrees, 
the total proper motion $\mu$ derived from the {\it UCAC2} catalogue (Zacharias et al., 2004), 
and {\it Tycho-2} (H{\o}g et al., 2000) and {\it 2MASS} (Cutri et al., 2003) colour indices. 
The electronic version of the IBVS contains direct links to the {\it ASAS3} source data.


\begin{table}
\normalsize
\caption{Characteristics of the four new double mode RR Lyrae stars} \vskip2mm
\begin{center}
\begin{tabular}{ccccc} 
\hline
Star                 & GSC~4868-0831 & GSC~7411-1269 & GSC~8403-0647 & GSC~8936-2145 \\
\hline
$V_{ASAS3}$          & 10.45$-$11.05   & 11.95$-$13.00   & 12.05$-$12.80   & 12.25$-$13.25 \\
HJD Maximum          & 2452940.85    & 2452789.60    & 2453108.83    & 2452614.70 \\
Period F (d)         & 0.56391(11)   & 0.46126(7)    & 0.46781(6)    & 0.51721(9) \\
Period 1O (d)        & 0.42079(6)    & 0.34247(4)    & 0.34778(4)    & 0.38521(5) \\
Period ratio         & 0.7462(2)     & 0.7425(1)     & 0.7434(1)     & 0.7448(2) \\
$R_{21}$(F)          & 0.11(4)       & 0.20(2)       & 0.17(4)       & 0.23(3) \\
$R_{21}$(1O)         & 0.21(2)       & 0.18(2)       & 0.19(4)       & 0.14(2) \\
$\Phi_{21}$(F)       & 4.00(19)      & 4.10(8)       & 3.91(23)      & 3.95(10) \\
$\Phi_{21}$(1O)      & 4.90(7)       & 4.82(4)       & 5.05(11)      & 4.68(4) \\
Amplitude ratio 1O/F & 2.45(10)      & 0.98(2)       & 1.05(6)       & 1.37(5) \\
$G_{1,1}$            & 4.26(8)       & 4.01(2)       & 4.28(10)      & 3.99(9) \\
$G_{-1,1}$           & 3.68(8)       & 3.88(7)       & 3.35(21)      & 3.86(5) \\
$b$                  & +23.8	     & $-$11.8         & $-$30.7         & $-$17.0 \\
$\mu$ (mas/yr)       & 54.6(1.7)     & 36.2(2.3)     & 13.7(3.0)     & 28.2(4.6) \\
$(B-V)_T$            & 0.41          & $-$0.30         & $-$0.07         & $-$ \\
$J-K_s$              & 0.30(3)       & 0.30(4)       & 0.23(4)       & 0.31(3) \\
\IBVSshortlink{http://www.astrouw.edu.pl/\~gp/asas/asas.html}{ASAS3} &
\IBVSshortlink{http://www.astrouw.edu.pl/cgi-asas/asas_lc/084747-0339.1}{084747-0339.1} &
\IBVSshortlink{http://www.astrouw.edu.pl/cgi-asas/asas_lc/183952-3200.9}{183952-3200.9} &
\IBVSshortlink{http://www.astrouw.edu.pl/cgi-asas/asas_lc/195612-5043.7}{195612-5043.7} &
\IBVSshortlink{http://www.astrouw.edu.pl/cgi-asas/asas_lc/081610-6644.8}{081610-6644.8} \\
\hline
\end{tabular}
\end{center}
\end{table}


GSC~4868-0831 is the brightest RRd star known thus far.  The amplitude of the first overtone 
is much higher than that of the fundamental mode for this star.  
For the other stars the amplitudes of both modes are more alike.


For GSC~7411-1269 the additional cross-coupling frequencies
$2f_0 + f_1$, $f_0 + 2f_1$ and $2f_0 + 2f_1$ are present in the power spectrum.


GSC~8403-0647 is the Southern component of a close pair separated by $11\arcs$,
which the {\it ASAS3} camera cannot resolve.  
The {\it ASAS3} average position is in fact about midway between the two stars, 
but slightly closer to GSC~8403-0647 (40\% of the distance), so that the latter is most probably
the brightest of the two in $V$.  This is confirmed by their {\it UCAC2} magnitudes.  
The {\it 2MASS} catalogue indicates that the companion 
is slightly brighter in near infrared wavelengths
($J = 11.94$ compared to $J = 12.01$), but is also much redder with $J-K_s = 0.62$. 
Due to its redness, it is unlikely that the companion is the RR Lyrae variable.
The pair is most probably physically unrelated, as shown by their {\it UCAC2} proper motions.


The plots in Figs.~1 to 8 give for each of the stars the phase diagram for the fundamental mode and the first overtone mode,
in both cases prewhitened
for the other mode and its harmonics.

\IBVS2fig{7cm}{5593-f1.eps}{ASAS3 phased light curve for the fundamental period of GSC~4868-0831.}{5593-f2.eps}{ASAS3 phased light curve for the first overtone period of GSC~4868-0831.}
\IBVSfigKey{5593-f1.eps}{GSC 4868-0831}{light curve}
\IBVSfigKey{5593-f2.eps}{GSC 4868-0831}{light curve}

\IBVS2fig{7cm}{5593-f3.eps}{ASAS3 phased light curve for the fundamental period of GSC~7411-1269.}{5593-f4.eps}{ASAS3 phased light curve for the first overtone period of GSC~7411-1269.}
\IBVSfigKey{5593-f3.eps}{GSC 7411-1269}{light curve}
\IBVSfigKey{5593-f4.eps}{GSC 7411-1269}{light curve}

\IBVS2fig{7cm}{5593-f5.eps}{ASAS3 phased light curve for the fundamental period of GSC~8403-0647.}{5593-f6.eps}{ASAS3 phased light curve for the first overtone period of GSC~8403-0647.}
\IBVSfigKey{5593-f5.eps}{GSC 8403-0647}{light curve}
\IBVSfigKey{5593-f6.eps}{GSC 8403-0647}{light curve}

\IBVS2fig{7cm}{5593-f7.eps}{ASAS3 phased light curve for the fundamental period of GSC~8936-2145.}{5593-f8.eps}{ASAS3 phased light curve for the first overtone period of GSC~8936-2145.}
\IBVSfigKey{5593-f7.eps}{GSC 8936-2145}{light curve}
\IBVSfigKey{5593-f8.eps}{GSC 8936-2145}{light curve}


{\bf Acknowledgements:} 
This research has utilised the {\it ASAS3} 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.
John Greaves is acknowledged for helpful suggestions and comments.


\newpage

\references

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Kholopov P.N., et. al., 2003, General Catalogue of Variable Stars version 1.4 Vol. IV, {\tt http://www.sai.msu.su/groups/cluster/gcvs/gcvs/}

H{\o}g E., Fabricius C., Makarov V.V., Urban S., Corbin T., Wycoff G., Bastian U., Schwekendiek P., Wicenec A., 2000, {\it A\&A}, {\bf 355}, L27

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Zacharias N., Urban S.E., Zacharias M.I., Wycoff G.L., Hall D.M., Monet D.G., Rafferty T.J., 2004, {\it AJ}, {\bf 127}, 3043

\endreferences

\end{document}


