\documentstyle[twoside, epsfig]{article}

\input ibvs2.sty

\begin{document}

\IBVShead{5501}{3 February 2004}

\IBVStitle{The Beat Cepheids NSV~6728, GSC~8607-0608, EY~C\lowercase{ar} and BE~P\lowercase{up}}

\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), Liga Iberoamericana de Astronom\'\i a (LIADA); e-mail: varsao@fullzero.com.ar}

\SIMBADobj{GSC 8607-0608}
\SIMBADobj{GSC 8691-1294}
\SIMBADobj{EY Car}
\SIMBADobj{BE Pup}
\GCVSobj{NSV 06728}
\GCVSobj{EY Car}
\GCVSobj{BE Pup}

\IBVStyp{ CEP(B) }
\IBVSkey{Variable stars, Cepheids, Beat Cepheids, Double mode variability}
\IBVSabs{Analysis of the publicly available ASAS3 data has lead to the}
\IBVSabs{discovery of four new Galactic beat Cepheids: }
\IBVSabs{NSV~6728, GSC~8607-0608, EY~Car and BE~Pup are pulsating in the}
\IBVSabs{fundamental and first overtone modes.}

\begintext

\vspace*{-1cm}

The online edition of the General Catalogue of Variable Stars (GCVS, Kholopov et.al., 2003)
lists sixteen Galactic double mode or beat Cepheids (GCVS type CEP(B) or DCEP(B)). 
In addition to these sixteen stars, DZ~CMa (Berdnikov and Turner, 1998)
and HD~304373 (Beltrame and Poretti, 2002), have been found only recently to be beat Cepheids.
Two other stars, V371~Per (Schmidt et.al., 1995) and BE~Pup (Hacke and Richert, 1989), 
are suspected of being double mode pulsators (Welch, 1998).
Only CO~Aur and HD~304373 are pulsating in the first and second radial overtones, 
all other stars pulsate in the fundamental (F) and first overtone (1O) modes. 

From the publicly available ASAS3 survey data (Pojmanski, 2002), we found that also
NSV~6728, GSC~8607-0608 and EY~Car are double mode Cepheids, pulsating in F and 1O modes.  
In addition, we could confirm the double mode nature of BE~Pup, so that the number of 
known Galactic beat Cepheids has now risen to 22.
The source data for BE~Pup, GSC~8607-0608, EY~Car and NSV~6728 can be found at the ASAS3 home page.
The electronic version of the IBVS contains direct links.
\IBVSshortlink{http://www.astrouw.edu.pl/cgi-asas/asas_lc/073335-2550.7}{BE~Pup} 
\IBVSshortlink{http://www.astrouw.edu.pl/cgi-asas/asas_lc/100914-5714.6}{GSC~8607-0608} 
\IBVSshortlink{http://www.astrouw.edu.pl/cgi-asas/asas_lc/104223-6110.0}{EY~Car}
\IBVSshortlink{http://www.astrouw.edu.pl/cgi-asas/asas_lc/143655-5815.7}{NSV~6728} 
Fundamental data about the four stars are listed in Table~1.  
It includes values for the invariant Fourier parameters and for the generalized phase difference $G_{1,1}$ 
of the cross coupling term $f_0 + f_1$ as defined by Poretti and Pardo (1997).  
Formal errors are given between parentheses in units of the last significant decimal. \\

\begin{table}[b]
\vspace*{-1cm}
\caption{Characteristics of the four new double mode Cepheids} \vskip2mm
\begin{tabular}{ccccc} 
\hline
 & NSV~6728 & GSC~8607-0608 & EY~Car & BE~Pup \\
\hline
V Range & 9.75-10.35 & 10.70-11.40 & 9.90-10.45 & 13.0-13.9 \\
HJD Maximum & 2452082.85 & 2452655.94 & 2451891.71 & 2452981.78 \\
Period F (d) & 4.317(7) & 4.089(3) & 2.876(3) & 2.870(3) \\
Period 1O (d) & 3.037(3) & 2.869(4) & 2.036(2) & 2.048(2) \\
Period ratio (1O/F) & 0.7035(14) & 0.7017(11) & 0.7077(8) & 0.7136(8) \\
$R_{21}$(F) & 0.21(1) & 0.18(2) & 0.30(2) & 0.39(12) \\
$R_{21}$(1O) & 0.11(2) & 0.07(2) & 0.16(7) & 0.46(25) \\
$\Phi_{21}$(F) & 4.40(3) & 4.20(7) & 4.01(5) & 3.96(34) \\
$\Phi_{21}$(1O) & 4.97(7) & 5.12(14) & 4.60(44) & 3.44(41) \\
Amplitude ratio (1O/F) & 0.61(1) & 0.72(2) & 0.22(2) & 0.49(13) \\
$G_{1,1}$ & 4.31(5) & 4.35(6) & 3.90(6) & 3.38(35) \\
\hline
\end{tabular}
\end{table}

NSV~6728 (= BV~447 = GSC~8691-1294 = CPD$-$57~6710) was suspected of variability by 
Strohmeier et al. (1964).  
Phase plots for the fundamental period and for the first overtone, after prewhitening
for the fundamental period and three harmonics, are presented in Figs.~1 and 2.  

\medskip

GSC~8607-0608 (= CPD$-$56~2913 = TDS~7069) is noted as either a fundamental or first overtone Cepheid
by Pojmanski (2004).  It is a double star (TDSC~28377, Fabricius et.al., 2002) with a separation of
0\farcs9 and a primary star with $V_T = 11.44$ and $B_T - V_T = 1.26$, 
and a secondary with $V_T = 11.86$ and $B_T - V_T = 1.17$.
Because ASAS3 cannot resolve this pair, it is impossible
to determine which of the components is variable.  Because of the period ratio of $0.7017 \pm 0.0011$,
it is however highly unlikely that both components are normal single-mode Cepheids, rather than one being a beat
Cepheid.
The proper motions of the components given by TDSC are identical, so that it could be a physical pair.  
Y~Car, a spectroscopic binary (B\"ohm-Vitense et al., 1997), has been the only known beat Cepheid 
in a multiple system (Szabados, 2003).
The phase plots from ASAS3 data for GSC~8607-0608 are given in Figs.~3 and 4.

\medskip

EY~Car (= GSC~8957-1924 = HIP~52380) had been suspected of being a beat Cepheid before,
based on (sparse) data from Mitchell et al. (1964).
It was therefore observed by Pike and Andrews (1979), and later also by
Mantegazza and Poretti (1992), but they failed to detect the first overtone pulsation.
Since these authors observed the star for only three and two weeks respectively, and
the first overtone period of EY~Car turns out to be close to two days, the phase coverage
was poor.  Combined with the small amplitude of the first overtone variation,
this explains why these authors did not find it.
The star is also near the limiting magnitude of Hipparcos (ESA, 1997), so that it is not surprising
this instrument did not reveal a secondary frequency.
The phase plots from ASAS3 data are given in Figs.~5 and 6.

\medskip

BE~Pup is not catalogued in the GSC version 1, its GSC 2.2 position is 
$\alpha_{2000}= 07\hr33\mm35\fsec486$, $\delta_{2000}= -25\deg50\arcm37\farcs18$. 
It is near the faint limit for accurate observations of ASAS3.  
The errors in Table~1 and the scatter in the phase plots
in Figs.~7 and 8 are therefore larger than for the other three stars.
BE~Pup was classified as a possible population~II object (GCVS type CWB:) 
by Fernie and Hube (1968), based on a deduced distance of $400~pc$ from the Galactic plane.  
However, no spectroscopic observations are available to confirm this.  
Fernie and Hube (arbitrarily) chose 400~pc as the lower limit for population~II objects.  
BD~Pup, close to BE~Pup, 
but at a distance of 390~pc from the Galactic plane, was classified as a population~I Cepheid. 

The fundamental periods of BE~Pup and EY~Car are almost the same, but their period ratios
are significantly different.  This indicates that there is no simple relation between
the fundamental period and the period ratio, but an other parameter such as
metallicity is involved as well (Morgan and Welch, 1997).  
A plot of the period ratio against
period for all known Galactic double mode (1O/F) Cepheids is given in Fig.~9.  
For previously known beat Cepheids, data from Welch (1998) have been used (points).  
The stars discussed in this paper are denoted by crosses.
Fig.~10 plots the values of the generalized phase difference $G_{1,1}$ for the cross coupling
term $f_0 + f_1$.  Values are taken from Poretti and Pardo (1997), supplemented with 
values derived from the ASAS3 data for some additional stars.
The lowest value (with the largest error bar) is for BE~Pup.

\IBVS2fig{6cm}{5501-f1.eps}{ASAS3 phased light curve for the fundamental period of NSV~6728.}{5501-f2.eps}{ASAS3 phased light curve for the first overtone period of NSV~6728 after prewhitening with the fundamental period and harmonics.}
\IBVSfigKey{5501-f1.eps}{NSV 6728}{light curve}
\IBVSfigKey{5501-f2.eps}{NSV 6728}{light curve}

\IBVS2fig{6cm}{5501-f3.eps}{ASAS3 phased light curve for the fundamental period of GSC~8607-0608.}{5501-f4.eps}{ASAS3 phased light curve for the first overtone period of GSC~8607-0608 after prewhitening with the fundamental period.}
\IBVSfigKey{5501-f3.eps}{GSC 8607-0608}{light curve}
\IBVSfigKey{5501-f4.eps}{GSC 8607-0608}{light curve}

\IBVS2fig{6cm}{5501-f5.eps}{ASAS3 phased light curve for the fundamental period of EY~Car.}{5501-f6.eps}{ASAS3 phased light curve for the first overtone period of EY~Car after prewhitening with the fundamental period and harmonics.}
\IBVSfigKey{5501-f5.eps}{EY Car}{light curve}
\IBVSfigKey{5501-f6.eps}{EY Car}{light curve}

\IBVS2fig{6cm}{5501-f7.eps}{ASAS3 phased light curve for the fundamental period of BE~Pup.}{5501-f8.eps}{ASAS3 phased light curve for the first overtone period of BE~Pup after prewhitening with the fundamental period and harmonics.}
\IBVSfigKey{5501-f7.eps}{BE Pup}{light curve}
\IBVSfigKey{5501-f8.eps}{BE Pup}{light curve}

\IBVS2fig{6cm}{5501-f9.eps}{Plot of the 1O/F period ratio versus fundamental period for Galactic beat Cepheids.}{5501-f10.eps}{Plot of the phase difference $G_{1,1}$ versus fundamental period for 1O/F pulsators.}


{\bf Acknowledgements:} 
This research has utilised the ASAS3 public photometry catalogue and the SIMBAD and VizieR 
databases operated at the {\it Centre de Donn\'ees Astronomiques (Strasbourg)} in France.
The authors are grateful to John Greaves for helpful discussions and to Paul Van Cauteren for
providing access to the Mitchell et al. paper.

\vspace*{-0.2cm}

\references

Beltrame, M., Poretti, E., 2002, {\it A\&A}, {\bf 386}, L9

Berdnikov, N., Turner, D.G., 1998, {\it IBVS}, No.\,4567 

B\"ohm-Vitense, E., Evans, N.R., Carpenter, K., Morgan, S., Beck-Winchatz, B., Robinson, R., 1997, {\it AJ}, {\bf 114}, 1176

ESA, 1997, The Hipparcos and Tycho Catalogues, ESA SP-1200

Fabricius, C., Hog, E., Makarov, V.V., Mason, B.D., Wycoff, G.L., Urban, S.E., 2002, {\it A\&A}, {\bf 384}, 180

Fernie, J.D., Hube, J.O., 1968, {\it AJ}, {\bf 73}, 492

Hacke G., Richert M., 1989, {\it Astron. Tsirk.}, {\bf 1539}, 14

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/}

Mantegazza, L., Poretti, E., 1992, {\it A\&A}, {\bf 261}, 137

Mitchell, R.I., Iriarte, B., Steinmetz, D., Johnson, H.L., 1964, {\it Bol.Obs.Ton.Tac.}, {\bf 3}, 153

Morgan, S.M., Welch, D.L., 1997, {\it AJ}, {\bf 114}, 1183

Pike, C.D., Andrews, P.J., 1979, {\it MNRAS}, {\bf 187}, 261

Pojmanski, G., 2002, {\it Acta Astronomica}, {\bf 52}, 397

Pojmanski, G., 2004, {\it Preprint}, astro-ph/0401125, %\\{\tt http://xxx.lanl.gov/abs/astro-ph/0401125}

Poretti, E., Pardo, I., 1997, {\it A\&A}, {\bf 324}, 133

Schmidt, E.G., Chab, J.R., Reiswig, D.E., 1995, {\it AJ}, {\bf 109}, 1239

Strohmeier, W., Knigge, R., Ott, H., 1964, {\it IBVS}, No.\,62 

Szabados, L., 2003, {\it IBVS}, No.\,5394 ({\tt http://www.konkoly.hu/CEP/intro.html})

Welch, D., 1998, {\tt http://dogwood.physics.mcmaster.ca/Cepheid/BeatCepheid.html}

\endreferences

\end{document}

