

\documentstyle[twoside, epsf]{article}

\input ibvs2.sty

\begin{document}
\IBVShead{5252}{6 April 2002}

\IBVStitletl{Observations of Outside-eclipse Brightness}{ Variations of CM~D\lowercase{ra}}


\IBVSauth{Kozhevnikov, V.P.$^1$; Kozhevnikova, A.V.$^2$}

\IBVSinst{Astronomical Observatory, Ural State University, Lenin Av. 51, Ekaterinburg, 620083, Russia,\\ e-mail:valerij.kozhevnikov@usu.ru }
\IBVSinst{Astronomy Department, Ural State University, Lenin Av. 51, Ekaterinburg, 620083, Russia,\\ 
e-mail:alla125@mail.ru }


\SIMBADobj{CM Dra}
\IBVStyp{ BY }
\IBVSkey{photometry, star spot}

\begintext

CM Dra is a spotted eclipsing binary star with a period of 1.27 days. Strassmeier et al. (1993) 
reckoned this star among the BY Dra type variables and included it in 
their catalogue of chromospherically active binary stars. The BY~Dra stars 
and their cousins, the RS~CVn stars, often show
brightness variations caused by star spots. Such variations in CM~Dra  were observed
by Lacy (1977). These variations proved to be sine wave with an
amplitude of about 0.02 mag. Probably, because of their low amplitude these 
variations were
not described in the literature after 1977. In the present paper we report results 
regarding the outside-eclipse brightness variations of CM~Dra from
our observations obtained at Kourovka observatory of the Ural State University
during 1996--97. 

Some particular characteristics of CM~Dra, mainly small sizes of the components (their 
total surface area is about 12\% of the solar value (see Lacy 1977, for all system elements)) 
make this star suitable in order to search for extrasolar planets by means of
differential photometry. However, such a task needs a large amount of observations. 
To obtain 
sufficient observational coverage, the ``TEP'' (Transits of Extrasolar Planets) network 
was formed with the participation of several observatories (Deeg et al. 1998, Doyle 
et al. 2000). At Kourovka observatory we performed 
the CM Dra observations as a part of the TEP network observations. We used the 70 cm 
telescope and the two-star photometer (Kozhevnikov \& Zakharova 2000). The observations 
were made through a standard $R$ filter. The total duration of our observations 
obtained during 43 nights is 155 hours.

Besides the main goal of these observations, i.e. detecting of transits 
of planets orbiting in the plane of binary components, we decided to 
construct a composite lightcurve of CM~Dra in order to derive some 
orbital elements of the system. It seemed to be not a very simple task 
because the differential lightcurves showed slow airmass-related changes 
from differential extinction due to the large colour difference between CM~Dra and the 
comparison star. These slopes of the differential
lightcurves in the TEP network observations were 
removed by subtraction of a polynomial fit to the off-eclipse lightcurves 
(Deeg et al. 1998).
In order to construct the composite lightcurve, using the data obtained at 
Kourovka observatory, we decided not to remove the slopes of the individual 
lightcurves, hoping that they would be averaged owing to the large amount of 
the observations (155 hours). Besides the extinction, the slopes can contain 
brightness variations caused by star spots if they are present on the binary 
components. These brightness variations have to occur over approximately the 
same time-scale as the extinction since the period of CM~Dra's components is 
very likely locked to the binary period of 1.27 days. CM~Dra is a very  old system (see, for example, Viti et al. 1997), and its
components have to be  completely in synchronous rotation.  Thus, having not 
removed the slopes of the individual lightcurves, we could detect 
outside-eclipse 
brightness variations of CM~Dra. Unfortunately, during almost two years of our 
observations the instrumental colour system underwent little changes 
that resulted in the average differential magnitudes somewhat different for different 
observing seasones. These average differential magnitudes were previously 
subtracted from the individual lightcurves belonging to the different observing 
seasons.


\IBVSfig{16cm}{5252-f1.eps}{Composite lightcurve of CM~Dra. The lower frame
shows an expanded view of the outside-eclipse variations. The solid curve is 
the best-fitting sine wave. Two periods are shown for the sake of clarity}


\vskip 1cm


The composite lightcurve obtained by averaging all our observations of CM Dra 
is shown in Fig.~1. We used 856 bins per phase. The duration
of the phase bins equals the exposure time (128 seconds). Each point is the 
average magnitude of 4--6 individual measurements obtained in different nights. 
The outside-eclipse
brightness variations with an amplitude of about 0.02 mag are clearly visible in
this lightcurve. As mentioned above, similar sinusoidal
variations of the off-eclipse lightcurve of CM Dra with the same amplitude
were found by Lacy (1977). However, there is a remarkable distinction between the
results obtained by Lacy in 1976 and our results.
The phase of maximum brightness that was obtained twenty years 
prior to our observations was 0.28, whereas the phase of maximum brightness in our 
observations is 0.60. The difference is approximately equal  to one third of
the orbital period.


Since our observations cover almost two years, we tried to find the difference 
of the phases of maximum brightness between 1996 and 1997. In order to make this, we 
constructed two composite lightcurves, using the data of 1996 and 1997 separately. 
Then we found the phases of maximum brightness, using sine wave fits to these 
lightcurves. Unfortunately, as seen in Fig. 1, the points around the sine wave 
are not distributed smoothly. In order to obtain more realistic values of formal 
errors from least squares technique, we averaged points of the lightcurves 
in 0.02 phase bins. Then sine waves were fitted to 
these lightcurves consisting of 50 points. The results with their rms errors 
are given in Table~1.
As follows from Table~1, the difference of the phases of maximum 
brightness for the two years only slightly exceeds the triple interval of the rms 
errors, and 
we cannot draw a conclusion about the appreciable change of the phases of 
maximum brightness between 1996 and 1997. On the contrary, we can conclude that 
these phases 
can change significantly within one decade or more (as follows from the comparison of
our observations and the observations made by Lacy (1977)). 


\vskip 1cm

\centerline{Table 1. The information about the best-fitting sine waves}
\vskip 3mm
\begin{center}
\begin{tabular}{ccccc}
\hline
time    & number & duration & semiamplitude & phase of \\
interval & of nights    & (hours)  &  (magnitude)  & maximum  \\
\hline
1996 April--October & 22  & 68 & $0.010\pm0.002$ & $0.517\pm0.023$  \\
1997 April--October & 21  & 87 & $0.013\pm0.001$ & $0.635\pm0.009$  \\
all the data               & 43  & 155 & $0.012\pm0.001$ & $0.597\pm0.008$  \\
\hline
\end{tabular}
\end{center}

\vskip 1cm




Brightness variations of the BY~Dra and RS~CVn stars are usually 
considered to be the effect of star spots that covered appreciable 
parts of the surfaces of one or both components of the system. 
The strong resemblance of the shapes and amplitudes of such variations in CM~Dra that 
were observed in 1976 (see Fig. 5 of Lacy 1977) and in 1996--97 seems 
amazing and may mean that these brightness variations are caused by the same 
feature, namely, a spotted region on the star. 

As seen in Fig.~1, the brightness variations in CM~Dra have a smooth shape  
without a flat top or bottom.  As noted by Lacy (1977), such a shape of 
brightness variations may mean that the spotted area is located very 
close to the rotational pole of one of the stars, since otherwise it would 
be out of view at some point in the orbit. Another hint that the spotted 
area in CM~Dra is located close to the rotational pole may be the low amplitude of the 
brightness variations. As far as one can see in the data given by Strassmeier et al. 
(1993) in the catalogue of chromospherically active binary stars, brightness 
variations due to star spots in the BY Dra stars have usually greater amplitudes than 
in CM Dra and can reach several tenth of a magnitude. If the spotted area 
in CM Dra is similar to the spotted areas in most of the BY Dra stars, 
then by assuming the location of the star spot almost strictly on the rotational pole 
one can easily account for the small amplitude of the brightness variations due to  
a slightly nonsymmetric shape of the star spot. Also the change of the phases of
maximum brightness between 1976 and 1996--97 can be explained, supposing a little 
change in shape or asymmetry of this large polar star spot after 1976.  


{\it Acknowledgements.} The authors thank H.~Deeg and L.~Doyle for helpful comments.


\references

Deeg, H.J., Doyle, L.R., Kozhevnikov, V.P., Martin, E., Oetiker, B., 
Palaiologou, E., Schneider, J., Afonso, C., Dunham, E.T., Jenkins, J.M., 
Ninkov, Z., Stone, R., Zakharova, P.E., 1998, {\it A\&A},
{\bf 338}, 479 


Doyle, L.R., Deeg, H.J., Kozhevnikov, V.P., Oetiker, B., Martin, E.L., 
Blue, J.E., Rottler, L., Stone, R.P.S., Ninkov, Z., Jenkins, J.M., 
Schneider, J., Dunham, E.W., Doyle, M.F., Paleologou, E., 2000, {\it ApJ},
{\bf 535}, 338 


Kozhevnikov, V.P., Zakharova, P.E., 2000, {\it ASP Conf. Ser.}, {\bf 219}, 381,
in: {\it Proc. Euroconference on Disks, Planetesimals and Planets}, 
ed. Garzon, F., Eiroa, C., de Winter, D., \& Mahoney, T.J.
%Changed: Fri Jun  3 10:29:53 MEST 2005

Lacy, C.H., 1977, {\it ApJ}, {\bf 218}, 444 


Strassmeier, K.G., Hall, D.S., Fekel, F.C., Scheck, M., 1993, {\it A\&AS},
{\bf 100}, 173 

Viti, S, Jones, H.R.A., Schweitzer, A., Allard, F., Hauschildt, P.H.,
Tennyson, J., Miller, S., Longmore, A.J., 1997, {\it MNRAS},
{\bf 291}, 780 

\endreferences


\end{document}

