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\IBVShead{5632}{4 July 2005}

\IBVStitletl{ASAS 081933-2358.2: RR\lowercase{c}-Type Variable}{ with Two Closely Spaced Frequencies}

\IBVSauth{Antipin, S. V.$^{1,2}$; Jurcsik, J.$^3$}

\IBVSinst{Sternberg Astronomical Institute, 13, University Ave.,
Moscow 119992, Russia; e-mail: antipin@sai.msu.ru}
\IBVSinst{Institute of Astronomy, Russian Academy of Sciences,
48, Pyatnitskaya Str., Moscow 119017, Russia}
\IBVSinst{Konkoly
Observatory of the Hungarian Academy of Sciences, P.O. Box 67,
H-1525 Budapest, Hungary; e-mail: jurcsik@konkoly.hu}


\SIMBADobj{TYC 6556 609 1}

\IBVStyp{  RRc with two closely spaced frequencies  }

\IBVSkey{stars: double frequency variability - stars: variables:
RRc}

\IBVSabs{Electronically available photometry of}
\IBVSabs{ASAS 081933-2358.2 allow us to classify the variable}
\IBVSabs{as a RRc-type star with two closely spaced frequencies}
\IBVSabs{- the first known field star of this kind in our Galaxy.}


\begintext

According to the ASAS3 list of new variables (Pojmanski, 2003),
ASAS 081933-2358.2 (= TYC 6556 609 1 = CPD$-$23\deg3633 =
CD$-$23\deg7130) is an RRc-type variable with the period of 0.285668
days. However, this star deserves a more careful investigation
because of the exceptionally large scatter of its phased light curve. 
The classification of variable stars in the ASAS3 project is good enough 
in general, but in some cases important details are lost.
For example,  a circumspect analysis of ASAS3 data led to the 
discovery of four new double-mode Cepheids (Wils \& Otero, 2004).

The results of the frequency analysis of ASAS3 observations of
ASAS 081933-2358.2 are presented in Fig.\,1. It is found, that the
scatter of the light curve can be explained by the superposition
of two close frequencies corresponding to the following light
elements:
$$
JD_{max} =2453440.665 + 0\fday285665 \times E \,\,{\rm and} \\
$$
$$
JD_{max} = 2453015.749 + 0\fday296111 \times E.
$$
The phased light curves are shown in Fig.\,2.
Folded curves of the deviations from the mean
light curve of the other oscillation are plotted
in the bottom panels.

After prewhitening the data with the two main frequencies 
the residual spectrum (bottom panel in Fig.\,1)
indicates that the combination frequency ($f_1+f_2$)
also appears in the spectrum. 
The existence of nonlinear interaction between the oscillations
rules out the possibility that two  
RRc-type variables in the same line of sight (i.e., a blend) are observed.
According to the duplicity flag of the Tycho data
no indication of duplicity was found, which also strengthens our 
solution.

Closely spaced frequency components of three RRc stars in 
the globular cluster M55 were first discovered by Olech et al. (1999).
This type of variation (showing frequency doublets in the 
Fourier spectrum) was classified as $\nu_1$ variables by 
Alcock et al., (2000) utilizing the discovery of 
24 additional similar objects in the LMC.
Based on the large frequency ratio of the components (larger than 0.9 in every 
case) the most plausible explanation of the phenomenon is the excitation of 
a nonradial mode pulsation in the vicinity of the radial mode frequency.

\newpage

Up to now, no galactic field RRc stars showing similar behaviour 
has been discovered. This gives an unique opportunity, that by studying 
this relatively close object we could learn
much more about this interesting type of stars. 
 

\IBVSfig{13cm}{5632-f1.eps}{The power spectra.}
\IBVSfigKey{5632-f1.eps}{ASAS 081933-2358.2}{periodogram}


{\bf Acknowledgements:} The authors acknowledge the use Dr. G.
Pojmanski's ASAS3 observations. One of the authors (S. Antipin) is
grateful to the Russian Foundation of Basic Research (grant No.
05-02-16688) for partial support of this research. J. Jurcsik
acknowledges the support of the Hungarian OTKA Grant No 43504.

\newpage

\IBVSfig{12cm}{5632-f2.eps}{ASAS 081933-2358.2. The phased light
curves based on ASAS3 observations.}
\IBVSfigKey{5632-f2.eps}{ASAS 081933-2358.2}{phased light curve}


\references

Alcock C., Allsman R., Alves D.R., et al., 2000, {\it  ApJ}, {\bf
542}, 257

Olech A., Kaluzny J., Thompson I.B., Pych W., Krzeminski W.,
Shwarzenberg-Czerny A., 1999, {\it AJ}, {\bf 118}, 442

Pojmanski G., 2003, {\it  Acta Astronomica}, {\bf 53}, 341

Wils P., Otero S.A., 2004, {\it IBVS}, No. 5501

\endreferences

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