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\input ibvs2.sty

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\IBVShead{4502}{31 July 1997}

\IBVStitletl{PHOTOMETRIC OBSERVATIONS OF THE PRIMARY MINIMUM}{ OF THE ACTIVE ALGOL BINARY RZ CASSIOPEIAE}

\IBVSauth{Shin-ya NARUSAWA$^1$, Kikuichi ARAI$^2$, Kazuo NAGAI$^3$, Shigeo OHMORI$^4$,}
\IBVSauth{Mitsugu FUJII$^5$, Takeshi YASUDA$^6$}
\IBVSinst{Nishi-Harima Astronomical Observatory, Sayo, Hyogo, 679-53 Japan
E-mail: narusawa@nhao.go.jp}
\IBVSinst{Hanyu-Jitsugyo Senior High school, Hanyu, Saitama, 348 Japan}
\IBVSinst{Chigasaki, Kanagawa, 253 Japan}
\IBVSinst{Sugao Junior High School, Kawasaki, Kanagawa, 216 Japan}
\IBVSinst{Kurashiki, Okayama, 713 Japan}
\IBVSinst{Himeji Municipal Accommodation and Facilities Center, Himeji, Hyogo, 671-22 Japan}

\IBVSabs{Photoelectric and CCD observations of the primary minimum of the active
}
\IBVSabs{Algol binary RZ Cas were carried out. The observed times of the primary
}
\IBVSabs{minima are estimated and some of the observed light curves are presented.}

\SIMBADobj{RZ Cas}
\IBVStyp{ EA/SD }
\IBVSkey{photometry}

\begintext

    The active Algol binary RZ Cas has been well-known for its irregular 
period variations and also for the unusual changes of the shape of light 
curves 
at the bottom part of the primary minimum.  Concerning the period variation, 
Heged\"{u}s {\it et al.\/}\ (1992) proposed a possibility of the light-time effect 
and the apsidal motion, but Kiss {\it et al.\/}\ (1995) denied it according to their 
photometric observations.  Narusawa {\it et al.\/}\ (1994) suggested the similarity 
of activities between the RZ Cas and RS CVn stars.  Light changes within 
the primary minimum normally provides light curves of the partial-eclipse 
type.  However, some observers have reported that occasionally flat bottom 
(which resembles a total eclipse) appeared at the primary minimum (e.g., 
Nakamura {\it et al.\/}\ 1991, Heged\"{u}s {\it et al.\/}\ 1992, Narusawa {\it et al.\/}\ 1994 and 
Dumont 1995).  The cause of this phenomenon and its relation to the period 
variation have not yet been made clear.

    Some systematic photometry of RZ Cas was planned and carried out at 
various places in Japan to monitor its photometric behaviour.  Here the 
results of the observations performed at five places in Japan from January 
1995 to December 1996 are reported.  The photoelectric photometry with PMT 
was made at three observatories and the CCD photometry was done at two 
observatories as listed in Table 1.  At all the places the $V$ or $R$ colour 
filter similar to the standard Johnson-Kron-Cousins system was used. 
HR\,791 was employed as the comparison for the photoelectric observations, 
which is the same star as was used by Narusawa {\it et al.\/}\ (1994).  Two nearby 
stars, GSC\,4317-1437 and GSC\,4317-1578, were chosen as the comparison 
and check stars for the CCD observations, which were previously used by 
Narusawa \& Okyudo (1993).

     The estimated moments and the shapes of the light curves of the primary 
minima are given in Table 2.  The E and O$-$C values were calculated from 
the following ephemeris in Narusawa {\it et al.\/}\ (1994).

\vspace {4mm}
\ \ \ \ \ \ \ \ \ \ \ \ \ $ Min I = HJD\,2448960.2122 + 1\fday 1952572 \times E$ \hfill (1)
\vspace {4mm}
\vspace{2mm}
\IBVSfig{8cm}{4502-f1.ps}{
Light curve at the primary minimum of 21 Oct.\ 1995 (HJD\,2450012) observed by Nagai}

\IBVSfig{8cm}{4502-f2.ps}{
Light curve at the primary minimum of 27 Oct.\ 1995 (HJD\,2450018) observed by Arai}

\IBVSfig{8cm}{4502-f3.ps}{
Light curve at the primary minimum of 21 Dec.\ 1995 (HJD\,2450073) observed by Arai}

     A probable orbital-period change occurred during the period HJD 2448220 
and HJD 2448581 (Narusawa {\it et al.\/}\, 1994).  The ephemeris (1) was derived with 
the observed times after HJD\,2448581.  The O$-$C values in Table 2 indicate
no apparent period change and the orbital period has been surely constant
from HJD\,2448581 to HJD\,2450446 (about 5 years).

    Some of the observed light curves are shown in Figures 1-3.  As seen in 
Table 2, we did not observe the flat bottom in the primary minimum during 
our observations.  It is necessary to continue the photometric observations 
to clarify the relation between period changes and light curve variations.

\vspace {4mm}
\begin{table}
\caption{Observational instruments}
\begin{center}
\begin{tabular}{lll}
\hline
Observer  &  Telescope(I)  &  Camera/Detector    \\   
\hline
\\
Ohmori &     40cm  Cs$^{\dagger}$  &   PMT    (Hamamatsu 1P21)  \\    
Arai   &     28cm  SC  &   PMT    (Hamamatsu R1414) \\        
Fujii  &     28cm  SC  &   CCD    (SBIG ST6)   \\         
Nagai  &     20cm  SC  &   PMT    (Hamamatsu R647-04) \\
Yasuda &     60cm  Cs$^{\star}$ &   CCD    (SBIG ST6)    \\        
\hline
\end{tabular}
\\
(I) Cs:Cassegrain reflector, SC:Schmidt Cassegrain telescope\\
$^{\dagger}$:Science Museum of Kawasaki City\\
$^{\star}$:Nishi-Harima Astronomical Observatory\\
\end{center}
\end{table}

\vfill\eject
\vspace {4mm}
\begin{table}
\caption{Estimated times and shapes of the observed primary minima}
\begin{center}
\begin{tabular}{llllll}
\hline
HJD  &  E(I) & O$-$C(I) & Filter & Shape(II) & Observer \\
2400000+     &         &     (day)   &        &         & \\ 
\hline
\\
49723.9824   &    639  &  $+$0.0008  &    V   &    P   &    Arai   \\ 
49723.983    &    639  &  $+$0.001   &    V   &   $-$  &    Ohmori \\
49743.106    &    655  &  $+$0.000   &    V   &   $-$  &    Ohmori \\
49988.136    &    860  &  $+$0.003   &    V   &   $-$  &    Yasuda \\
49994.1123   &    865  &  $+$0.0026  &    R   &    P   &    Fujii  \\
50012.0401   &    880  &  $+$0.0016  &    V   &    P   &    Nagai  \\
50018.0157   &    885  &  $+$0.0009  &    V   &    P   &    Arai   \\
50023.9920   &    890  &  $+$0.0009  &    V   &    P   &    Arai   \\
50023.9931   &    890  &  $+$0.0020  &    R   &   $-$  &    Fujii  \\
50031.1652   &    896  &  $+$0.0025  &    R   &   $-$  &    Fujii  \\
50072.9967   &    931  &  $+$0.0000  &    V   &   $-$  &    Arai   \\
50078.9728   &    936  &  $-$0.0001  &    V   &    P   &    Arai   \\
50445.921    &   1243  &  $+$0.004   &    V   &   $-$  &    Nagai  \\
\hline
\end{tabular}
\\
(I) The E and O$-$C values are calculated from the ephemeris (1)\\
(II) P:Partial, $-$:Uncertain\\
\end{center}
\end{table}

  We would like to acknowledge Dr.\ Y.\ Nakamura at Fukushima University for
useful comments.  We are also grateful to Mr.\ H.\ Akazawa, Mr.\ Y.\ Ito, Mr.\ N.\ Ohkura and Mr.\ E.\ Suzuki who helped this work.
This investigation was supported in part by a Science Research Grant (B)
No.\ 08914028 from the Ministry of Education, Science, Sports and Culture 
of Japan.

\vspace{10mm}
\references

Dumont, M., 1995, {\it GEOS Note Circ.}, No.\,NC {\bf 776}, 1

Heged\"{u}s, T., Szatm\'{a}ry, K., \& Vink\'{o}, J.\ 1992, {\it Ap\&SS}, {\bf 187}, 57

Kiss, L.L., G\'{a}l, J., \& Kasz\'{a}s, G.\ 1995, {\it IBVS}, No.\,4181

Nakamura, Y., Narusawa, S., \& Kamada, M.\ 1991, {\it IBVS}, No.\,3641

Narusawa, S., \& Okyudo, M.\ 1993, {\it Annu.\ Report Nishi-Harima Astron.\ Obs.}, {\bf 3}, 11

Narusawa, S., Nakamura, Y., \& Yamasaki, A.\ 1994, {\it AJ}, {\bf 107}, 1141

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