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\input ibvs2.sty

\begin{document}

\IBVShead{5011}{29 December 2000}

\IBVStitle{1994 BV Photoelectric Observations of CG C\lowercase{yg}}

\IBVSauth{Dapergolas, A.$^1$; Kontizas, E.$^1$; Kontizas, M.$^2$}

\IBVSinst{National Observatory of Athens, Institute of Astronomy and Astrophysics, 
I. Metaxa and Vas.\ Paulou,\\ \indent Palaia Penteli, GR--152\,36, Athens, Greece,
e-mail: adaperg@astro.noa.gr; ekonti@astro.noa.gr}
\IBVSinst{Section of Astrophysics Astronomy and Mechanics, Department of Physics,
University of Athens,\\ \indent GR--157\,83, Athens, Greece, email: mkontiza@cc.uoa.gr}

\SIMBADobj{CG Cyg}

\IBVStyp{ RS CVn - type binary star  }
\IBVSkey{eclipsing binaries}

\IBVSabs{In this study, data obtained from 1994}
\IBVSabs{photometric observations of CG Cygni are presented and}
\IBVSabs{the new light elements are given.}

\begintext

Photometric observations for CG Cyg have been reported previously by
Dapergolas et al.\ (1994); Heckert (1994); Zeilik et al.\ (1994) and
references therein. The star is among the most peculiar ones of the
short-period chromospherically active binary stars. As noted by
Dapergolas et al.\ (1994) the system presents irregularities outside of
the eclipses and the depths of the minima changes with time. All these
suggests that CG~Cyg is a complex system, with changing active regions,
due probably to photospheric activity.

CG Cyg was observed for the period 4--16 August 1994 with the 1.2-m
Kryonerion telescope and a single channel photon counting photometer
described by Dapergolas and Korakitis (1987). The photometer employs a
high gain 9789QB phototube and conventional $B$, $V$ filters. Its
output is fed to a microcomputer enabling rapid data access. The data
reduction method is the standard one and as a comparison star
BD\,+34\deg4216 was used. The constancy of the comparison star was
verified by Milone et al.\ (1979). The data presented here were
obtained with an accuracy of $\pm0\fmm015$. Table~1 lists the dates
of observations and phases covered and number of points obtained. Our
observations where nearly simultaneous with those of Heckert (1994).
%\IBVSindexobj{BD +34 4216}

In Table~2 the times of minima and the $O-C$ values are listed for the
$V$ and $B$ bands, respectively. Times of minima are calculated
using the method described by Kwee and van Woerden (1956) whereas the
$O-C$ values were determined from the linear ephemeris ${\rm
HJD_{min\,I}} = 2439425.1221 + 0\fday631141 \times E$, given by Milone
and Ziebarth\ (1974).

\begin{table}
\begin{center}
\caption{Dates of observations and phases covered}
\begin{tabular}{ccccc}
\hline
JD & \lower6pt\hbox{Date} & \lower6pt\hbox{Phase} & \multicolumn2c{Points} \\[-3pt]
\cline{4-5}
2440000\,+ & & & $V$ & $B$ \\
\hline
9569 & 04 August 1994 & .77 .12 & 95 & 81 \\
9570 & 05 August 1994 &.36 .78 & 113 & 110 \\
9572 & 07 August 1994 &.56 .84 & 92 & 93 \\
9577 & 12 August 1994 &.45 .88 & 111 & 113 \\
9578 & 13 August 1994 &.01 .48 & 134 & 132 \\
9579 & 14 August 1994 &.67 .09 & 112 & 112 \\
9581 & 16 August 1994 &.78 .14 & 92 & 91 \\
\hline
\end{tabular}

\vskip36pt

\caption{Types and times of minima}
\begin{tabular}{ccccccc}
\hline
& & \multicolumn2c{$V$ colour} & & \multicolumn2c{$B$ colour} \\
\cline{3-4}\cline{6-7}
Date & Type & HJD & \lower6pt\hbox{$O-C$} & & HJD & \lower6pt\hbox{$O-C$} \\[-3pt]
& & 2440000\,+ & & & 2440000\,+ & \\
\hline
 4/8/1994 & I & $9569.4836\pm.0002$ & 0.051 & & $9569.4837\pm.0001$ & 0.051 \\
 5/8/1994 & II & $9570.4324\pm.0012$ & 0.554 & & $9570.4323\pm.0004$ & 0.554 \\
12/8/1994 & II & $9577.3740\pm.0006$ & 0.553 & & $9577.3742\pm.0005$ & 0.553 \\
13/8/1994 & I & $9578.3197\pm.0003$ & 0.051 & & $9578.3199\pm.0001$ & 0.051 \\ 
14/8/1994 & I & $9579.5821\pm.0001$ & 0.051 & & $9579.5821\pm.0001$ & 0.051 \\
16/8/1994 & I & $9581.4758\pm.0002$ & 0.052 & & $9581.4758\pm.0002$ & 0.052 \\
\hline
\end{tabular}

\vskip36pt

\caption{CG Cyg spot parameters}
\begin{tabular}{lcr@{.}lr@{.}l}
\hline
& & \multicolumn2c{$V$ band} & \multicolumn2c{$B$ band} \\
\hline
Longitude & $\lambda_{1}$ & 276 & $7\pm8.0$ & 286 & $6\pm7.6$ \\ 
Latitude & $\beta_{1}$ & 56 & $2\pm33.6$ & 62 & $1\pm20.3$ \\
Radius & $\gamma_{1}$ & 10 & $4\pm4.8$ & 12 & $0\pm4.5$ \\
$\chi^{2}$ & & 84 & 6 & 87 & \\ 
\hline
\end{tabular}

\vskip36pt

\caption{CG Cyg clean parameters}
\normalsize
\begin{tabular}{ccccccccc}
\hline
Filter & $L_{1}$ & $k=r_2/r_1$ & $\Delta\theta_{0}$ & $r_{1}$ & $i$ (deg) & $L_{2}$ & $\chi^{2}$ \\
\hline
$V$ & $0.689\pm.030$ & $0.936\pm.056$ & $-18.37\pm0.1$ & $0.240\pm.006$ & $82.2\pm.4$ & $0.288\pm.046$ & 80.5 \\
$B$ & $40.707\pm.023$ & $0.956\pm.045$ & $-18.37\pm0.1$ & $0.237\pm.005$ & $82.1\pm.3$ & $0.273\pm.004$ & 80.8 \\
\hline
\end{tabular}
\end{center}
\end{table}

The data are modeled using the Information Limit Optimization Technique
(ILOT) described by Budding and Zeilik (1987). The main assumptions of
the ILOT are: (1)~the activity in general is most apparent in
the hotter (primary) star, (2) maculation effects are separable
from proximity and eclipse effects, and (3) a cool circular spot
model adequately represents the key parameters of stellar magnetic
activity--these are longitude, latitude, size (area), and temperature
of the active (spotted) regions.

\IBVSfig{20.0cm}{5011-f1.ps}{Initial, clean and spot fits. Phases are given in degrees}

\vskip-11pt
The derived residuals from the initial fit of Fig.~1a are plotted in
Fig.~1c. Then into the distortion wave a circular spot of 0~K
temperature was fitted. The results are seen in Table~3. The fits are
performed independently for both colours $V$ and $B$. The results of
the spot fitting agree, within the errors, to those found by Zeilik
et al.\ (1994) and Heckert (1994). It is found that the spot for CG~Cyg
tend to cluster in Active Longitude Belt, around the $270\deg$ as it
is noticed by Zeilik et al.\ (1994). We tested several times to fit a
second spot with fixed latitude at $45\deg$, as it is described by
Heckert (1994), but the results were very uncertain. This result
means that our data sample with $S/N \approx 75$ is unable to detect
small spots. This is in agreement with a series of verification tests
of the ILOT programs reported by Rhodes et al.\ (1990). However, a small
distorsion in the observed light curve of the star is seen toward the
first quadrature of the system (see spot fit in Fig.~1c). This
distorsion was more evident a few days later when the star was
observed from Mt.~Laguna with the 61-cm telescope (Heckert 2000) and
clearly showed a second spot.

The results of spot fitting where inserted in the initial fit model and
a clean fits was made (Fig.~1b). So the distortion wave was removed and
the clean parameters are seen in Table~4. The values of $L_{1}$, $k$,
$r_{1}$, $i$ (deg), and $L_{2}$ agree with those found by Zeilik et
al.\ (1994) and Heckert (1994). From the results presented in Table~4
assuming constant inclination $i$ and phase correction
$\Delta\theta_{0}$ the mass ratio $q=m_{2}/m_{1}$ of the system can be
derived. For $V$ colour and for $B$ the values of mass ratio are
$q=0.571\pm0.108$ and $q=0.647\pm0.089$ respectively. These values are
in agreement with those found by Heckert (1994) and Popper (1993), and
strengthen the hypothesis that the mass ratio of CG~Cyg is lower than
1.0.

From our data set it seems that CG~Cyg changes its spot
structure rather rapidly and probably is one of the most active RS CVn
type binary system.

The authors would like to acknowledge the Greek General Secretariat of
Research and Technology for financial support and one of the authors,
D.~A., wishes to express his sincere thanks to Dr.~P.~Heckert for his
helpful suggestions.
 
\references

Budding, E., and Zeilik, M., 1987, {\it Astrophys.\ J.}, {\bf 319}, 827

Dapergolas, A., Kontizas, E., and Kontizas, M., 1994, {\it IBVS},
No.\ 4051

Dapergolas, A., and Korakitis, R., 1987, {\it Pub.\ Nat.\ Obs.\ of Athens},
Series~II, No.\ 28

Heckert, P., 1994, {\it IBVS}, No.\ 4127

Heckert, P., 2000, private communication

Kwee, K.K., and Van Woerden, K.E., 1956, {\it
Bull.\ Astr.\ Inst.\ Netherlands}, {\bf 12}, 327

Milone, E.F., and Ziebarth, K.E., 1974, {\it
Publ.\ Astron.\ Soc.\ Pacific}, {\bf 86}, 684

Milone, E.F., Castle., K.G., Swadrom, D., Burke, E.W., Hall, D.S.,
Michlovic, J.E., and Zissell, R.E., 1979, {\it Astron.\ J.}, {\bf 84},
417

Popper, D.M., 1993, {\it Astrophys.\ J.}, {\bf 404}, L67

Rhodes, M., Budding, E., and Zeilik, M., 1990, {\it ASP Conf. Ser.}, {\bf 9}, 252, in {\it Cool stars,
stellar systems, and the Sun}, Proceedings of the 6th Cambridge
Workshop, Astr.\ Soc.\ of the Pacific
%Changed: Fri Jun 17 11:42:17 MEST 2005

Zeilik, K., Gordon, S., Jaderlund, E., Ledlow, M., Summers, D.L.,
Heckert, P.A., Budding, E., and Banks, T.S., 1994, {\it
Astrophys.\ J.}, {\bf 421}, 303


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