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\IBVShead{6038}{29 October 2012}

\IBVStitletl{Final classification of the bright variable star WW~CM{\lowercase {a}}}

\IBVSauth{SZABADOS, L.}

\IBVSinsto{Konkoly Observatory, 
Research Centre for Astronomy and Earth Sciences,
Hungarian Academy of Sciences, 
H-1121 Budapest, Konkoly Thege M. str. 15-17.
Hungary\\
 email: szabados@konkoly.hu}
 
\SIMBADobj{WW CMa}
 
\IBVStyp{  EA  }
\IBVSkey{eclipsing binary, photometry, period change}

\begintext 


\IBVSabs{Following the discovery of its brightness variability, the bright} 
\IBVSabs{(<V> = 9.2m) variable star WW CMa was classified}
\IBVSabs{as an RR Lyrae type, then a Cepheid type, and during a half century} 
\IBVSabs{an unstudied variable star with rapid light changes.}
\IBVSabs{The ASAS photometric data clearly indicate its eclipsing binary}
\IBVSabs{nature with an orbital period of 2.5163489 days.}

The brightness variability of the star BD~$-21^{\circ}$~1424 was 
discovered by Hoffmeister (1933). This star preliminarily designated
as 176.1932 was characterised with a short period in the discovery
note. Florja (1937a) observed the star visually but his 42 brightness
estimates obtained on 21 nights (in 1933 and 1935) were inconclusive:
in Florja (1937b) he remarked `that confirmation of variability is
still required'. Parenago (1939), however, determined a period of
5.7660 days based on the analysis of Florja's visual data, and
without explicitly stating that 176.1932 CMa is a Cepheid, the 
English translation of the Russian title of this communication 
(On the period of 15 Cepheids) implies this classification.
Interestingly enough, Joy (1937) already included this variable
among his sample in his seminal study of radial velocity of Cepheids
in spite of the fact, that no hint at the Cepheid nature of this
star was available in the literature at that time. Soon after, a
new suggestion emerged: Kanda (1938) classified this star as an 
RR~Lyrae type variable with a period of 0.499355 day. 

The photographic magnitudes based on the Dushanbe (Stalinabad) 
plate archive, however, contradicted both period values (Erleksova, 
unpublished). Therefore, though the compilers of the first edition 
of the General Catalogue of Variable Stars classified this star 
(designated as WW~Canis Maioris in the meantime) as a Cepheid 
variable characterised with Parenago's (1939) elements 
(Kukarkin \& Parenago 1948), in the subsequent editions of the GCVS 
no definite type of variability has been assigned to this rather 
bright star, though the editors mention rapid variations in its 
brightness.

While it was classified among classical Cepheids, several studies
involved WW~CMa in the Cepheid sample investigated from statistical
point of view. The first reliable photoelectric data published by 
Irwin (1961) were decisive: neither the shape, nor the amplitude
resembled typical Cepheids, in addition, the $B-V$ colour index
remains practically constant. The classification of this variable
star as a Cepheid became untenable.

Decades later, new reliable photometric data became available
thanks to the ASAS project (Pojmanski \& Maciejewski 2004).
The long and homogeneous observational series covering the
interval 2000-2009 clearly indicated that WW~CMa was originally
misclassified. In fact, it is an eclipsing binary, as is
correctly referred to in the ASAS on-line catalogue quoting
an orbital period of 2.5163 days. This value was refined by Sebastian 
Otero who determined a period of 2.51636 days from the same data
(VSX, 2010).

\begin{center}
\IBVSfig{9.0cm}{6038-f1.eps}{$O-C$ diagram of WW~CMa}
\IBVSfigKey{6038-f1.jpg}{WW CMa}{$O-C$ diagram}
\end{center}

To facilitate the correct classification of this bright variable
star, here I revise and improve the orbital period and discuss the 
available information on WW~CMa. The early and the modern photometric 
data have been subjected to an $O-C$ analysis. The ASAS data 
provide a reliable normal light curve so the normal maxima for 
earlier data have been determined by fitting Florja's (1937a) and 
Irwin's (1961) observations to this normal curve. Moreover, the ASAS 
data have been divided into annual segments. The resulting moments 
of normal maxima, the epoch, the $O-C$ value and its weight in the 
least squares fit, as well as the source of the observations are 
listed in Table~1. The tabulated $O-C$ values have been calculated 
by using the ephemeris:

$$ C = 2451925\fday8877 + 2\fday5163489 E .$$

This period has been obtained by a least squares fit to the
ASAS normal maxima. The $O-C$ diagram in Figure~1 clearly
indicates that the orbital period of WW~CMa is not stable on
the time scale of decades: between JD 2430000 - JD 2450000 it
was 2.5163 days, i.e., shorter than in the last decade.
The pattern of the $O-C$ values cannot be approximated with a
parabola implying a continuous period change, so the underlying
physical mechanism causing the instability of the orbital
period can be the episodic mass loss/exchange.

\begin{table}
\begin{center}
\caption{$O-C$ residuals of WW~CMa}
\begin{tabular}{lrccl}
\hline
\noalign{\vskip 0.3mm}
JD$_{\odot}$ & $E\ $ & $O-C$ & $W$ & Data source\\
2\,400\,000 + &&d&\\
\noalign{\vskip 0.3mm}
\hline
\noalign{\vskip 0.3mm}
27470.0369& $-$9719&  0.0603& 1& Florja (1937a)\\
35200.1131& $-$6647&  0.0629& 2& Irwin (1961)\\
51925.8963&     0&  0.0000& 3& ASAS \\
52205.2131&   111&  0.0075& 3& ASAS \\
52663.1671&   293& $-$0.0051& 3& ASAS\\
53020.4943&   435&  0.0075& 3& ASAS\\
53408.0095&   589&  0.0125& 3& ASAS\\
53755.2665&   727&  0.0201& 3& ASAS\\
54150.3382&   884&  0.0327& 3& ASAS\\
54474.9434&  1013&  0.0352& 3& ASAS\\
54784.4785&  1136&  0.0654& 3& ASAS\\
55083.9056&  1255&  0.0528& 1& ASAS\\
\noalign{\vskip 0.3mm}
\hline
\end{tabular}
\label{tab-wwcma-oc}
\end{center}
\end{table}

WW~CMa was a target in the sample of ASAS eclipsing variables
with the aim at searching for chromospheric activity (Parihar
et~al. 2009). They observed this binary star on JD\,2\,453\,267
and did not find any chromospheric emission. In addition,
Parihar et~al. (2009) determined the spectral types of
the components from the 2MASS colour indices of WW~CMa 
($J-H=0.31$ , $H-K=0.08$ , $V-J=1.15$; Cutri et~al. 2003). 
The resulting spectral types are: G0V+G6V, i.e., both
components are nearly solar type stars.

\begin{center}
\IBVSfig{9.0cm}{6038-f2.eps}{Photometric phase curve based on the 
ASAS data}
\IBVSfigKey{6038-f2.jpg}{WW CMa}{photometric phase curve}
\end{center}

The eclipsing light curve folded on the accurate period of
2.5163489 days is plotted in Fig.~2 based on the ASAS photometry
(the outlying points have been omitted). 

\begin{center}
\IBVSfig{8.0cm}{6038-f3.eps}{Radial velocity phase curve based on
Joy's (1937) data}
\IBVSfigKey{6038-f3.jpg}{WW CMa}{radial velocity phase curve}
\end{center}

The radial velocity data published by Joy (1937) are plotted 
in Fig.~3. When plotting these data, the proper phase shift 
deduced from Fig.~1 has been taken into account but the same
period was used for folding the data as for the photometric
phase curve. The radial velocity phase curve is compatible
with the orbital effect. However, a new series of accurate
radial velocities is necessary to determine the orbital
elements of this binary system formed by solar type stars.

%%%%\textbf{Acknowledgements} 
This work has been supported by the
ESTEC Contract No.\,4000106398/12/NL/KML.

\references

Cutri R. M. et al. 2003, VizieR On-line Data Catalog: II/246

Florja N. F. 1937a, {\it Sternberg Publ.}, {\bf 8}, No.\,2, 106

Florja N. F. 1937b, {\it Perem. Zvezdy}, {\bf 5}, 104

Hoffmeister C. 1933, {\it Astr. Nachr.}, {\bf 247}, 281

Irwin J. B. 1961, {\it ApJS}, {\bf 6}, 253

Joy A. H. 1937, {\it ApJ}, {\bf 86}, 363

Kanda S. 1938, {\it Tokyo Bull.}, {\bf 284}

Kukarkin B. V. \& Parenago P. P. (eds.) 1948, General Catalogue
  of Variable Stars, 1st Edition, Moscow

Parenago P. P. 1939, {\it Perem. Zvezdy}, {\bf 5}, 280

Parihar P., Messina S., Bama P., Medhi B.~J., Muneer S.,
  Velu C., Ahmad A. 2009, {\it MNRAS}, {\bf 395}, 593

Pojmanski G. \& Maciejewski G. 2004, {\it AcA}, {\bf 54}, 153

VSX 2010, http://www.aavso.org/vsx/index.php?view=detail.top\&oid=5198

\endreferences

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