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\begin{document} 

\IBVShead{4509}{14 August 1997} 
 
\IBVStitle{BVRI Observations of an eclipse of RZ C\lowercase{as}} 
 
\IBVSauth{JEFFREY HILL, WAYNE OSBORN} 
\IBVSinsto{Physics Department, Central Michigan Univ., 
Mt.\ Pleasant, MI 48858, U.S.A.} 
\vskip-2mm 
\IBVSinsto{\small e-mail: osborn@dune.phy.cmich.edu\large} 

\IBVSabs{BVRI photometry of an eclipse of the RZ Cas is reported.}

\SIMBADobj{RZ Cas}
\IBVStyp{ EA/SD }
\IBVSkey{photometry, CCD}
 
\begintext 
 
The Algol-type eclipsing binary RZ Cas is an interesting 
system that has both x-ray (McCluskey and Kondo 1984) and 
radio (Drake {\it et al.}\ 1986) emission.  The variable is 
bright (V$_{max}$ = 6.2) with a relatively short period 
(1\fday 195) and deep primary eclipse \hbox{($\Delta $V =1\fmm 5)} 
and therefore has often been observed photometrically. 
Nevertheless, the system is still not well understood. 
Particularly puzzling is the change in shape of light curve 
at primary minimum.  It is accepted that the eclipses are 
partial (Heged\"us {\it et al.}\ 1991, Maxted {\it et al.}\ 
1994, Narusawa {\it et al.}\ 1994), but some primary minima 
exhibit a flat-bottom light curve characteristic of a total 
eclipse.  Recently published examples can be found in 
Arganbright {\it et al.}\ (1988), Heged\"us (1989), Nakamura 
{\it et al.}\ (1991), and Narusawa {\it et al.}\ (1994). 
 
Continued minimum timings and photometric monitoring at a variety of 
wavelengths are needed to determine the cause of the light curve 
variations.  This note presents BVRI observations of a recent primary 
eclipse.  The data were obtained with the 40-cm Cassegrain reflector 
of the Brooks Astronomical Observatory at Central Michigan University 
employing a Photometrics Star-1 CCD camera and standard 
Johnson-Cousins BVRI filters.  Details concerning our CCD system, 
observing techniques, and data reduction methods have been given by 
Miller and Osborn (1996). 
 
Observations began with clear skies about one hour before the 
predicted time of minimum on 1996 December 12.  Sets of exposures with 
the four filters were made, alternating between the comparison star HD 
15784 = BD+67\deg 224 and RZ Cas.  Occasional observations were 
made of HD 16769 = BD+67\deg 215 as a check star.  Clouds appeared 
shortly after the eclipse minimum and affected the measures until the 
monitoring ended some 40 minutes later.  The exposure times for each 
filter were kept constant throughout the observations. 
 
Instrumental magnitudes were obtained by aperture photometry of the 
individual CCD frames.  Bias and flat field corrections were applied 
and then the star brightness measured using a circular aperture of 15" 
radius and a sky annulus with radii of 19" and 29".  The derived 
values were used to form differential magnitudes with respect to the 
comparison star.  The results are given Table 1, where the time is the 
fractional heliocentric Julian date after 2450425.0 and colons 
indicate observations in the presence of clouds.  Some measures 
obviously degraded by clouds have been discarded.  The measures of the 
check star indicate that the magnitudes have mean errors of about 0.02 
in B, 0.015 in V and less than 0.010 in R and I. \pagebreak 
 
\vskip 2mm 
 
\vskip 1cm 
 
\begin{table}
\caption{Differential magnitudes relative to HD 15784.}
\vskip -1mm 
\centerline{\figcap The integer part of the J.D.\ is 2450 425}
\vskip 1mm
\begin{tabular}{lclclclcl} 
\hline 
Star & Time &  $\Delta $B & Time &  $\Delta $V & Time &  $\Delta $R & Time & 
$\Delta $I \\ \hline 
RZ Cas   & \dotfill & \dotfill & .5564 & 0.271 & .5584 & 0.433 & .5592 & 0.534 \\ 
         & .5649 & 0.263 & .5654 & 0.469 & .5666 & 0.611 & .5669 & 0.677 \\ 
         & \dotfill & \dotfill & \dotfill & \dotfill & .5704 & 0.703 & .5707 & 0.763 \\ 
         & .5740 & 0.512 & .5745 & 0.687 & .5751 & 0.813 & .5763 & 0.858 \\ 
         & .5793 & 0.659 & .5799 & 0.844 & .5802 & 0.937 & .5805 & 0.950 \\ 
         & .5808 & 0.726 & .5813 & 0.888 & .5818 & 0.973 & .5821 & 0.981 \\ 
         & .5862 & 0.864 & .5867 & 1.024 & .5872 & 1.080 & .5875 & 1.076 \\ 
         & .5879 & 0.913 & .5884 & 1.057 & .5889 & 1.113 & .5894 & 1.096 \\ 
         & .5921 & 0.969 & .5927 & 1.103 & .5933 & 1.172 & .5936 & 1.148 \\ 
         & .5940 & 1.008 & .5948 & 1.139 & .5955 & 1.192 & .5958 & 1.162 \\ 
         & .5964 & 1.044 & .5970 & 1.175 & .5975 & 1.226 & .5977 & 1.188 \\ 
         & .6003 & 1.082 & .6007 & 1.185 & .6013 & 1.236 & .6018 & 1.185 \\ 
         & .6021 & 1.075 & .6026 & 1.188 & .6032 & 1.211 & .6037 & 1.168 \\ 
         & .6041 & 1.055 & .6046 & 1.173 & .6050 & 1.196 & .6053 & 1.153 \\ 
         & .6100 & 1.013 & .6106 & 1.094 & .6112 & 1.093 & .6114 & 1.117 \\ 
         & .6117 & 0.966 & .6122 & 1.038 & .6126 & 1.14: & .6129 & 1.11: \\ 
         & .6179 & cloud & .6184 & cloud & .6188 & cloud & .6192 & 0.92: \\ 
         & .6195 & 0.82: & .6201 & 0.86: & .6204 & cloud & .6209 & cloud \\ 
         & .6248 & 0.60: & .6255 & 0.75: & .6260 & 0.95: & .6264 & cloud \\ 
         & .6267 & cloud & .6272 & 0.71: & .6277 & 0.77: & .6279 & 0.79: \\ 
         & .6307 & 0.39: & .6312 & 0.59: & .6315 & 0.72: & .6319 & cloud \\ 
         & .6345 & 0.33: & .6350 & 0.51: & .6353 & 0.59: & .6357 & 0.65: \\ 
         &       &       &       &       &       &       &       &       \\ 
HD 16769 & .5845 & $-$.905 & .5850 & $-$.633 & .5854 & $-$.501 & .5857 & $-$.320 \\ 
         & .6082 & $-$.936 & .6086 & $-$.651 & .6091 & $-$.509 & .6095 & $-$.337 \\ 
         & .6233 & $-$.95: & .6238 & $-$.66: & .6241 & $-$.51: & .6244 & $-$.32: \\ \hline 
\end{tabular} 
\end{table} 
\vskip 1cm 
 
The resulting light curves are shown in Figure 1. While clouds 
affected the latter observations, it is clear that this eclipse does 
not show a flat bottom.  Other recent eclipses also do not show the 
flat-bottom effect (Davis 1996).  Our light curves also show some 
evidence for the ``light instabilities'' that have been noted by 
others (e.g.\ Olson 1982a, 1982b, Heged\"us 1989, Davis 1996).  The 
average from applying the method of bisecting chords to the four 
individual light curves yields the following time of minimum: 
 
\begin{center} 
JD$_{Helio} $ = 2450425.6004 $\pm $ 0.0002. 
\end{center} 
 
This research was made possible by support of the U.S.\ National 
Science Foundation through grant USE-9250926. 
 
\pagebreak
\bigskip 
\IBVSfig{15.5cm}{4509-f1.ps}{The differential light curves in BVRI (y-axis in mag)} 

\vskip -1cm
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\references 
 
Arganbright, D.V., Osborn, W., and Hall, D.S.\ 1988, {\it IBVS}, No.\ 
3224 
 
Davis, S.M.\ 1996, {\it Proceedings of the 1996 Keck Undergraduate 
Symposium on Research in Astronomy}, 29 
 
Drake, S.A., Simon, T., and Linsky, J.L., 1986, {\it AJ}, {\bf 91}, 1229 
 
Heged\"us, T.\ 1989, {\it IBVS}, No.\ 3381 
 
Heged\"us, T., Szatm\'ary, K., and Vink\'o, J., 1992, {\it Ap${\&}$SS}, {\bf 187}, 57 
 
Maxted, P.F.L., Hill, G., and Hilditch, R.W., 1994, {\it A${\&}$A}, {\bf 282}, 821 
 
McCluskey, G.E., and Kondo, Y., 1984, {\it PASP}, {\bf 96}, 817 
 
Miller, A., and Osborn, W., 1996, {\it IAPPP Communication}, No.\ 63, 40 
 
Nakamura, Y., Narusawa, S., and Kanada, M.\ 1991, {\it IBVS}, No.\ 3641 
 
Narusawa, S.Y., Nakamura, Y., and Yamasaki, A., 1994, {\it AJ}, {\bf 
107}, 1141 
 
Olson, E.C.\ 1982a, {\it ApJ}, {\bf 257}, 198 
 
Olson, E.C.\ 1982b, {\it ApJ}, {\bf 259}, 702 
 
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