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\IBVShead{4683}{10 March 1999}

\IBVStitletl{CL~Aurigae: a New Photometric Triple Star}

\IBVSauth{MAREK WOLF$^1$, LENKA \v{S}AROUNOV\'A$^2$, MIROSLAV BRO\v{Z}$^1$, ROBERT HORAN$^1$}

\IBVSinst{Astronomical Institute, Charles University Prague,
CZ - 180 00 Praha 8, V Hole\v{s}ovi\v{c}k\'ach 2, Czech Republic\\
E-mail: {\tt wolf@mbox.cesnet.cz}}

\IBVSinst{Astronomical Institute, Czech Academy of Sciences,
CZ - 251 65 Ond\v{r}ejov, Czech Republic}

\SIMBADobj{CL Aur}

\begintext
The detached eclipsing binary CL~Aurigae 
(=  BD +33\deg 0975 = GSC\,2393.1455 = HV\,6886 = FL\,439;
$\alpha_{2000} = 5\hr 12\mm 54\fsec 2$,
$\delta_{2000} = +33\deg 30\arcm 28\arcs$,
Sp.\ = A0, $V_{\rm max}$ = 11\fmm7)
is a photoelectrically neglected variable 
with a short orbital period of about 1.24 days. This system was selected 
as a possible candidate for the study of the apsidal motion 
(Heged\"us 1988) and thus it was also included to our observational 
project of eclipsing binaries with eccentric orbit 
(e.g.\ Wolf \& \v{S}arounov\'a 1995).

CL Aur was discovered to be a variable star photographically by
Hoffleit (1935). The first photographic light curve was obtained
by Kurochkin (1951), who also determined the first light elements:
$$
\hbox{Min.\ I} = \hbox{HJD\ }2\,432\,967.262 + 1\fday244\,366\,6 \times E.    
$$
Next visual observations were made by Szafraniec (1960),
the spectral type was determined by G\"otz \& Wenzel (1968).
The photographic plates of the GAIS (Nos.\,39052--40649 and Moscow observatories (Nos.\,16869--35540) were
examined by Fadeyev (1973).
Due to the relatively deep primary minimum this variable was often
observed visually, mostly by BBSAG observers (K.~Locher, H.~Peter,
R.~Diethelm). To our knowledge this star has not been measured 
photoelectrically since discovery.

\IBVSfig{8.5cm}{4683-f1.eps}{A plot of differential $R$ magnitudes
obtained during primary eclipse of CL~Aur on 14~January 1996
(circles).  The measurements of secondary minimum obtained on
22~November 1995 were shifted in time by +42.5 periods and are plotted
as triangles together with primary minimum.}


\vskip-11pt
Our new CCD photometry of CL~Aur was carried out during eight
nights between November 1995 and December 1998 at the
Ond\v{r}ejov Observatory, Czech Republic.
Primarily, a 65-cm reflecting telescope with a CCD-camera
(SBIG ST-6 or ST-8) was used. The measurements were done 
using the standard Cousins $R$ filter with 60 or 90 s exposure 
time. Two additional measurements were done using 
18-cm Maksutov-Cassegrain (Ond\v{r}ejov) and 25-cm Newtonian 
(Hradec Kr\'alov\'e) telescopes without filter.
The nearby stars GSC\,2393.1532 ($V = 11.4$~mag)
--- listed also as star $b$ by Kurochkin (1951) ---
and GSC\,2393.1435 ($V$ = 11.9 mag) on the same frame as CL~Aur
served as a comparison and check stars, respectively. Flat 
fields for the reduction of the CCD frames were routinely obtained
from exposures of regions of the sky taken at dusk or dawn.
Standard error of measurements varies from 0.01~mag to 0.02~mag.
The new moments of primary and secondary minimum and their errors
were determined using the least squares fit to the data, by
the bisecting chord method or by the Kwee--van Woerden algorithm.
These seven times of minimum are presented in Table 1. In this table, 
$N$ stands for the number of observations used in the calculation 
of the minimum time. The epochs were calculated according to the 
light elements of Kurochkin (1951).

Figure 1 shows the differential $R$ magnitudes during the primary
minimum observed at JD 2450097 (circles). Our measurements of
secondary minimum (triangles) were shifted exactly by 42.5 periods
(+52.8858 d) and are also plotted. The light amplitude
in $R$ colour for primary minimum according to our measurement is
$A_1 = 1.28$~mag, for secondary minimum we found $A_2$ = 0.32~mag.
The duration of both minima seems to be almost
identical, $D_1 \simeq D_2 \simeq$ 0.22 days = 0.176 phase.

\begin{table}
\begin{center}
\caption{New precise times of minimum of CL Aur}
\medskip
\begin{tabular}{lrlcl}
\hline
  JD Hel.   &   Epoch   &  Error  & $N$  &  \multicolumn{1}{c}{Instrument} \\
  24\,00000+   &           &  (days) &      &  \multicolumn{1}{c}{Filter}  \\   \hline
  50044.3958 &  13723.5  &  0.0007 &  63  &  65-cm, ST6, R \\
  50097.2826 &  13766.0  &  0.0002 &  56  &  65-cm, ST6, R \\
  50714.4888 &  14262.0  &  0.0006 &  52  &  18-cm, ST6, - \\
  50831.4589 &  14356.0  &  0.0001 &  39  &  65-cm, ST8, R \\
  50884.3431 &  14398.5  &  0.0003 &  68  &  65-cm, ST8, R \\
  51157.4815 &  14618.0  &  0.0006 &  24  &  25-cm, ST5, - \\
  51177.3914 &  14634.0  &  0.0001 &  51  &  65-cm, ST8, R \\  
\hline
\end{tabular}
\end{center}
\end{table}

\IBVSfig{8.5cm}{4683-f2.eps}{$O-C$ diagram of CL~Aur. The individual times of primary and secondary minimum are denoted by circles and triangles, respectively. Larger symbols are our CCD measurements. The curve corresponds to a third body orbit.}

\vskip-12pt
The change of period and possible apsidal motion of CL~Aur
were studied by means of an $O-C$~diagram analysis. We took into
consideration all visual and photographic measurements found in 
the literature as well as the CCD times given in Table 1. 
The $O-C$ graph for all moments of minimum are shown in Figure 2.
The photographic times or visual estimations obtained by
Kurochkin (1951) and BBSAG observers are also plotted.

We analysed the $O-C$~diagram and the light curve using the current
observations. Our results indicate that this binary has no 
significant eccentric orbit. According to our timings, the secondary 
minimum occurs at the phase $\Phi_{\rm II} = 0.5000 \pm 0.0001$
and the duration of primary and secondary eclipses is practically
identical. This system could be excluded
from the list of possible candidates for apsidal motion study.

Secondly, quasi-sinusoidal deviations of the $O-C$ values 
are remarkable after zero epoch and could be caused by 
a light-time effect. Preliminary analysis of the third body 
period gives the following parameters:

\begin{center}
\begin{tabular}{r@{\ =\ }l}
$P_3$ (period) & $8240 \pm 70$ days = 22.5 years  \\
$T_0$ (time of periastron) & J.D. $2\,443\,315 \pm 35$   \\
$A$ (semiamplitude) & $ 0.0166 \pm 0.0012$ day  \\
$e$ & $0.405 \pm 0.025 $ \\
$\omega$ & $179\deg \pm 2\deg$  \\
\end{tabular}
\end{center}

\noindent These values were obtained together with the new 
linear ephemeris 
$$
\begin{array}{r@{}r@{}l}
\hbox{Min.\ I} = \hbox{HJD\ }2\,432\,967.2472 & {} + 1\fday244\,371\,63 & {} \times E,\\
                                   \pm 0.0006 & \pm 0.000\,000\,17 \\
\end{array}
$$ 
by the least squares method. 
Assuming a coplanar orbit ($i_3 = 90\deg$) and a total mass
of the eclipsing pair $M_1 + M_2 \simeq 3 M_{\odot}$ (Harmanec 1988)
we can obtain a lower limit for the mass of the third component 
$M_{3, \rm min}$. The present explanation is supported by the quite
reasonable value of the mass function $f(M_3) = 0.0614 M_{\odot}$,
from which the minimum mass of the third body is $1M_{\odot}$. 

New high-accuracy timings of this interesting eclipsing system 
are necessary in the future in order to confirm and/or improve 
the light-time effect parameters given above.
For the current use we propose the following linear light elements:
$$
\hbox{Min.\ I} = \hbox{HJD\ } 2\,451\,177.3914 + 1\fday244\,365 \times E.
$$

{\it Acknowledgement.} 
This research has made use of the SIMBAD database, operated at CDS, 
Strasbourg, France.

\references

Fadeyev Ju.A., 1973, {\it Variable Stars Suppl.}\ {\bf 1}, 433

G\"otz W., Wenzel W., 1968, {\it Mitteilungen Ver\"anderliche
	Sterne} {\bf 5}, 5

Harmanec P., 1988, {\it Bull.\ Astr.\ Inst.\ Czech.}\ {\bf 39}, 329

Heged\"us T., 1988, {\it Bull.\ Inform.\ CDS} No.\,35, 15

Hoffleit D., 1935, {\it Harvard Bulletin} No.\,901, 20

Kurochkin N.E., 1951, {\it Variable Stars} {\bf 8}, 351

Szafraniec R., 1960, {\it Acta Astronomica} {\bf 10}, 99

Wolf M., \v{S}arounov\'a L., 1995, {\it Astron.\ Astrophys.\
      Suppl.\ Ser.}\ {\bf 114}, 143


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\IBVSerratum{4683}

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CL Aur is not BD +33\deg 0975.

\bigskip

The Editors

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