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\IBVShead{4379}{7 October 1996}

\title{PHOTOMETRIC INVESTIGATION OF Y CANUM VENATICORUM}

\begintext

The carbon star Y~CVn (BD +46\degr 1817 = HD\,110914,
$\alpha_{2000}$ = 12\fh 45\fmm 07\fss 8,
$\delta_{2000}$ = 45\degr 26\farcmm 25\farcss) was discovered as a variable star by Cannon (Pickering 1910).
Zverev (1936) was the first who determined a periodicity in its
variations described by the light elements:

$$T_{\max}=2426117+160.0\times E$$ 

The most systematic photometric investigation has been carried out by
Gaposhkin
who studied the star on plates of the Harvard Observatory
taken at the beginning  of this century. He has found the period
P = 157$\pm$23 days (Gaposhkin 1952). This is the
period
given in the General Catalogue of Variable Stars (Kholopov 1985).
Gaposhkin derived the mean maximum brightness
$8\fm 71\pm 0\fm 19$  and the mean minimum brightness
$9\fm 33\pm 0\fm 16$  and the extreme
maximum and minimum brightnesses 8\fm 18 and 10\fm 00 as well.

Nevertheless, the recent observations give evidence that mean period is
longer and the character of the light variations is somewhat more complicated.
The star was observed photoelectrically by Dzervitis and
Vetesnik from 1979 to 1982. Vetesnik (1983) estimated the period
of the star
P=251.8~days and new light elements for the times of the minimum:
$$T_{min}=2436097.5+251.8\times E$$

The BV photoelectric observations of Y~CVn presented in
this paper were performed by Jiri Papousek at the Brno
University Observatory in the course of the
years 1979--1994. The photometer attached to the 60 cm telescope
was equipped with an EMI 9656 photomultiplier and its filter
combinations ensured the measurements in the BV colours of
the Johnson standard photometric system. The data reduction method
was the standard one.  The comparison star and the check one was
HD\,110~834 and HD\,110~450, respectively.
The mean standard deviation of
one observation was better than 0.04~mag, most of them even
smaller than 0.007~mag. The total number of all observations was 138 in V
band and 135 in B band. The original data will be provided by the
author upon request.

The resulting light curves are plotted in Figures 1 and 2. The data used may
represent the longest continuous light curve of Y~CVn obtained
up to now.

The data shows that mean maximum brightness of Y~CVn
5\fm 35  and the mean minimum brightness 5\fm 61 in V band, while the corresponding values are 8\fm 33 and 8\fm 73 in B band. The
extreme maximum and minimum brightnesses are 5\fm 18 and 6\fm 02 in V band and
7\fm 97 and 9\fm 76  in B band. The colour index B$-$V varies
from 2\fm 73 to 3\fm 75.

After thorough treatment of data we have concluded:

\begin{itemize}
\item The period of light variations
determined by Date-Compensated Discrete Fourier Transform
(Kleczek 1987) is 267.8 days.
\vfill \eject

\centerline{2}
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\begin{center}
Figure 1. The light curve of Y~CVn in photometric band B between
the years 1979--1994. The data have been fitted by the
curve described by the formula (1).
\end{center}
\epsfysize=8cm
\centerline{\epsffile{4379-f2.ps}}
\begin{center}
Figure 2. The light curve of Y~CVn in photometric band V between
the years 1979--1994. The data have been fitted by the
curve described by the formula (1).
\end{center}
\vskip 5mm
\begin{center}
Table 1. The coefficients of the light curves (1)
\vskip 1mm
\begin{tabular}{lllllr}
\hline
band & $a_0$ [mag] & $a_1$ [mag] & $a_2$ [mag] & $a_3$[mag] &$\Delta\phi$\\
\hline
\\
 V~& 5.376$\pm0.014$ & 0.003$\pm0.002$ & 0.003$\pm0.001$ &
0.146$\pm0.009$ & $-0.027\pm0.011$\\
B & 8.261$\pm0.026$ & 0.011$\pm0.003$ & 0.007$\pm0.001$ &
$0.250$$\pm0.022$ & 0.025$\pm0.013$\\
\\
\hline
\end{tabular}
\end{center}
\vfill \eject

\centerline{3}
\vskip 1mm

\item There is a pronounced secular change.
\item Some additional irregularities typical for this type
of star are present.
\end{itemize}

The observed variations of Y CVn can be described by secular
change of the mean brightness, which may be represented by a part
of a parabola, superimposed on nearly sinusoidal variations of
constant amplitude and the period $P=267.8$ days.

Hence, the light curve in particular colour may be expressed by
the following formula with five parameters:
$$m=(a_0+a_1\,E+a_2\,E^2)+a_3\,\sin[2\pi (E+\Delta\phi)],
\eqno(1)$$ where $E=(JD_{hel}-2446458)/P$ and $\Delta\phi$
is a phase shift of the zero point of the sinusoidal component.
The coefficients have been found using a sophisticated least-squares
method (Mikulasek 1995). The coefficients found for B and V colours are presented in  Table~1.


As is seen from  Table~1, the phase shifts $\Delta\phi$ in
B a V band are not too different, so we could consider them to be
the same and being equal to zero. This conclusion
is quite natural from the physical point of view, too. So, we can
write  $$ m=a_0+a_1\,E+a_2\,E^2+a_3\,\sin(2\pi E) \eqno(2)$$

\noindent {\it Acknowledgement.} I am thankful to Jiri
Papousek for kindly providing me with his observations of Y~CVn and
Zdenek Mikulasek for his help with the treatment of data.

\vskip 1cm
\beginauthors
\authorsrightcol
Jiri DUSEK
Nicholas Copernicus Observatory
Kravi hora 2
CZ -- 616 00 Brno
Czech Republic
Internet: {\tt dusek@physics.muni.cz}
\endauthors


\references

Gaposhkin, S., 1952, {\it Ann. of Harv. Col. Obs.}, {\bf 118}, 22

Kholopov, P.N. et al., 1985, {\it General Catalogue of Variable
      Stars}, Nauka, Moscow

Kleczek, J., 1987, {\sl Exercises in Astronomy}, D.~Reidel
Publishing Co.

Mikulasek, Z., 1995, priv. comm.

Pickering, E.C., 1910, {\it HC}, No.\,159

Vetesnik, M., 1983, {\sl IBVS}, No.\,2271

Zverev, 1936, {\it Sternberg Publ.}, {\bf 8}, 1

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