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\IBVShead{5235}{12 February 2002}

\IBVStitletl{THE VARIABILITY OF THE ORBITAL PERIOD OF}{ RZ COMAE BERENICES}

\IBVSauth{Rovithis-Livaniou, H.$^1$; Rovithis, P.$^2$; Djura\v sevi\'c, G.$^3$}

\IBVSinst{Section of Astrophysics-Astronomy \& Mechanics, Dept. of Physics, Athens University,  Athens, Greece, \\ 
email: elivan@cc.uoa.gr}

\IBVSinst{National Observatory of Athens, Institute of Astronomy and Astrophysics, I. Metaxa and Vas.\ Paulou,\\ Palaia Penteli, GR--152\,36, Athens, Greece, e-mail: rovithis@astro.noa.gr}

\IBVSinst{Astronomical Observatory, Volgina 7, 11050 Belgrade, Yugoslavia, e-mail: gdjurasevic@aob.bg.ac.yu}

\SIMBADobj{RZ Com}
\IBVSkey{orbital period changes}

\begintext

The variable star RZ Com (BD\ +24${\deg}$ 2475) is a W UMa-type eclipsing binary that belongs to the W sub-class.  Although it is not a faint binary, ($V_{max}=10\fmm5$), it cannot be characterised as a well observed system. 
Its spectral classification is not clear, since:\\
1) in the old study by Struve \& Gratton (1948) the system is referred as K0;\\
2) Wood's et al. catalogue (1980) gives F7 for the primary and G0 for the secondary component, respectively;\\
3) Batten's et al. (1989) gives G2Vn; and\\   
4) in Hipparcos catalogue the system is referred to as G0Vn.


On the other hand, the more recent spectroscopic study of RZ Com (McLean \& Hilditch, 1983), does not give the spectral types of the two components.


From the $O-C$ diagram of the system, which is presented in Fig. 1, it  is obvious that its orbital period undergoes an obvious increase during the last two decades.

From all available data, given in Table 1, and from the weighted quadratic least squares fitting, we found:
$${\rm Hel.~JD(Min~I)} = 2434837.4211 + 0\fday33850481\times E + 3.69\times10^{-11} \times E^{2} $$
that yields to a period increase of the order of  $1\fday84 \times 10^{-11}$. 
This increase becomes a little greater $1\fday91 \times 10^{-11}$, if we do 
not take into account five points with very large scatter. These points are 
marked with asterisk in Table 1. 

The C values, in the $O-C$ differences of Table 1, have been calculated using Kholopov's (1985) ephemeris formula:
$${\rm Hel.~JD(Min~I)} = 2434837.4198 + 0\fday33850604 \times E$$

\IBVSedata{5235-t1.txt}

\IBVSfig{6cm}{5235-f1.eps}{The $O-C$ diagram of RZ Com, based on all available data, and to Moscow (1985) ephemeris formula. Squares have been used for photoelectric minima and small stars for the visual ones. The 5 visual points that were not taken into account are plotted with bigger stars.
}

\vspace*{-0.4cm}

According to some old studies (Broglia 1960, Aslan \& Herczeg 1984), the period
of RZ~Com was constant from 1934 till 1966, and equal to 0.3350604 days; while, the predominantly negative residuals from 1969 were perhaps indicating a 
change in the system's period (Aslan \& Herczeg 1984).  At the same time, 
Rovithis \& Rovithis-Livaniou (1984), proposed a new ephemeris with a slightly 
bigger value for the orbital period of RZ~Com. The present analysis not only 
confirms this, but also shows that this increase continues.\\
Most of the minima in Table 1, are visual; and although it is better to avoid 
using such data (because of their pure accuracy and usually large dispersion), 
in this particular case it is impossible to do otherwise, since the
photoelectric material is very poor.  New times of minimum light are needed to follow the orbital period behaviour of this interesting and rather neglected binary.\\

{\bf Acknowledgement:}
The authors thank Dr. K. Olah for her assistance. This work was partly 
supported financially by Athens University (grant No. 70/4/3305), and has made 
use of the Simbad database, operated at CDS, Strasbourg, France.\\

\vspace*{-0.7cm}
{\small
\IBVSEmin{
RZ Com & 34847.899      && I &&  $-$0.017 & Koch R.H., 1961. \\
       & 36341.4021     && I &&  0.0000    & Broglia P., 1960. \\
       & 40323.588      && I &&  0.001    & Baldwin M.E., 1973. \\
       & 40324.598      && I &&  $-$0.005 & Baldwin M.E., 1973. \\
       & 40338.638      && II && $-$0.013 & Baldwin M.E., 1973. \\
       & 40346.597      && I &&  $-$0.009 & Baldwin M.E., 1973. \\
       & 40347.617      && I &&  $-$0.004 & Baldwin M.E., 1973. \\
       & 40348.631      && I &&  $-$0.006 & Baldwin M.E., 1973. \\
       & 40369.621      && I &&  $-$0.003 & Baldwin M.E., 1973. \\
       & 40380.625      && II && $-$0.001 & Baldwin M.E., 1973. \\
       & 42571.420*     && II && $-$0.017 & Diethelm R., 1977. \\
       & 43213.413      && I &&  $-$0.000 & Peter H., 1977. \\
       & 43612.369*     && II && 0.026    & Peter H., 1979. \\
       & 43642.460      && II && $-$0.010 & Peter H., 1979. \\
       & 43656.512      && I &&  $-$0.006 & Peter H., 1979. \\
       & 44008.383      && II && $-$0.012 & Peter H., 1979. \\
       & 43663.455      && II && $-$0.002 & Peter H., 1979. \\
       & 43964.8906     && I &&  $-$0.0061 & Aslan Z. \& Herczeg T.J., 1984. \\
       & 43967.9372     && I &&  $-$0.0061 & Aslan Z. \& Herczeg T.J., 1984. \\
       & 43988.426      && II && 0.003    & Peter H., 1979. \\
       & 44214.711      && I &&  $-$0.003 & Locher K., 1980. \\
       & 44299.681      && I &&  0.002    & Locher K., 1980. \\
       & 44303.402      && I &&  $-$0.001 & Peter H., 1980. \\
       & 44316.435      && II && $-$0.000 & Locher K., 1980. \\}
}

{\small
\IBVSEmin{
RZ Com & 44339.449      && II && $-$0.005 & Peter H., 1980. \\
       & 44370.420      && I &&  $-$0.007 & Peter H., 1980. \\
       & 44382.428      && II && $-$0.016 & Peter H., 1980. \\
       & 44402.415      && II && $-$0.001 & Peter H., 1980. \\
       & 44595.701      && II && $-$0.009 & Locher K., 1981. \\
       & 44694.3712     && I &&  $-$0.0060 & Derman E. et al., 1982. \\
       & 44695.3868     && I &&  $-$0.0079 & Derman E. et al., 1982. \\
       & 45027.465      && I &&  $-$0.002 & Locher K., 1982. \\
       & 45046.4173     && I &&  $-$0.0062 & Rovithis \& Rovithis$-$Livaniou, 1984. \\
       & 45046.5878     && II && $-$0.0049 & Rovithis \& Rovithis$-$Livaniou, 1984. \\  
       & 45047.2652     && II && $-$0.0045 & Rovithis \& Rovithis$-$Livaniou, 1984. \\
       & 45047.4336     && I &&  $-$0.0054 & Rovithis \& Rovithis$-$Livaniou, 1984. \\
       & 45047.6027     && II && $-$0.0055 & Rovithis P. \& Rovithis$-$Livaniou E., 1984. \\     
       & 45049.2974     && II && $-$0.0034 & Rovithis \& Rovithis$-$Livaniou, 1984. \\
       & 45049.4673     && I &&  $-$0.0027 & Rovithis \& Rovithis$-$Livaniou, 1984. \\
       & 45061.327      && I &&  0.009    & Germann R., 1982. \\
       & 45076.363*     && II && $-$0.018 & Peter H., 1982. \\
       & 45100.416      && II && 0.001    & Peter H., 1982. \\
       & 45109.3832     && I &&  $-$0.0024 & Rovithis \& Rovithis$-$Livaniou, 1984. \\
       & 45110.3985     && I &&  $-$0.0026 & Rovithis \& Rovithis$-$Livaniou, 1984. \\
       & 45378.500      && I &&  0.002    & Locher K., 1983. \\
       & 45389.670      && I &&  0.001    & Locher K., 1983. \\
       & 45404.392      && II && $-$0.002 & Peter H., 1983. \\
       & 45493.409      && II && $-$0.012 & Germann R., 1983. \\
       & 45718.699      && I &&  0.002    & Locher K., 1984. \\
       & 45781.316      && I &&  $-$0.004 & Germann R., 1984. \\
       & 45815.341      && II && 0.003    & Germann R., 1984. \\
       & 45812.466      && I &&  0.003    & Peter H., 1984. \\
       & 45818.387      && II && 0.000    & Peter H., 1984. \\
       & 45884.399      && II && 0.004    & Peter H., 1984. \\
       & 46171.443      && II && $-$0.005 & Peter H., 1985. \\
       & 46180.428      && I &&  0.009    & Peter H., 1985. \\
       & 46535.340      && II && $-$0.002 & Germann R., 1986. \\
       & 46573.420      && I &&  $-$0.004 & Germann R., 1987. \\
       & 46908.372      && II && $-$0.004 & Germann R., 1987. \\
       & 46939.529*     && II && 0.030    & Peter H., 1987. \\
       & 46941.384      && I &&  0.004    & Peter H., 1987. \\
       & 47266.351      && I &&  0.005    & Germann R., 1988. \\
       & 47275.477      && I &&  $-$0.009 & Kirby G., 1991. \\
       & 47270.420      && I &&  0.012    & Peter H., 1988. \\
       & 47303.422      && II && 0.010    & Peter H., 1988. \\
       & 47323.392      && II && 0.008    & Blattler E., 1989. \\
       & 47592.341      && I &&  0.014 & Peter H., 1989. \\
       & 47612.305      && I &&  0.006 & Blattler E., 1989. \\
       & 47695.407      && II && 0.004 & Peter H., 1989. \\
       & 47946.421      && I &&  0.016 & Peter H., 1990. \\
       & 47969.432      && I &&  0.009 & Peter H., 1990. \\
       & 48001.426      && II && 0.014 & Peter H., 1990. \\
       & 48010.398      && I &&  0.016 & Peter H., 1990. \\
       & 48014.446      && I &&  0.002 & Peter H., 1990. \\
       & 48329.437      && II && 0.013 & Peter H., 1991. \\
       & 48348.397      && II && 0.016 & Peter H., 1991. \\
       & 48358.380      && I &&  0.013 & Peter H., 1991. \\
       & 48385.460      && I &&  0.013 & Peter H., 1991. \\
       & 48404.418      && I &&  0.015 & Peter H., 1991. \\
       & 48447.402      && I &&  0.008 & Peter H., 1991. \\
       & 48686.395      && I &&  0.016 & Peter H., 1992. \\
       & 48733.440      && I &&  0.009 & Peter H., 1992. \\
       & 48753.427      && I &&  0.024 & Peter H., 1992. \\}
}

{\small
\IBVSEmin{
RZ Com & 48795.396      && I &&  0.018 & Peter H., 1992. \\
       & 49076.361      && I &&  0.023 & Peter H., 1993. \\
       & 49097.350      && I &&  0.025 & Peter H., 1993. \\
       & 49147.450      && I &&  0.026 & Peter H., 1993. \\
       & 49167.412      && I &&  0.016 & Peter H., 1993. \\
       & 49416.547      && I &&  0.011 & Dedoch A., 1995a. \\
       & 49471.385      && I &&  0.009 & Peter H., 1994. \\
       & 49475.424*     && I &&  $-$0.012 & Peter H., 1994. \\
       & 49516.414      && I &&  0.018 & Paschke A., 1994. \\
       & 49537.391      && I &&  0.008 & Peter H., 1994. \\
       & 49793.482      && II && 0.019 & Dedoch A., 1995b. \\
       & 49810.410      && II && 0.022 & Martignoni M., 1996. \\
       & 49817.352      && I &&  0.024 & Peter H., 1995. \\
       & 49840.367      && I &&  0.021 & Peter H., 1995. \\
       & 49888.436      && I &&  0.022 & Peter H., 1995. \\
       & 50188.352      && I &&  0.022 & Peter H., 1996. \\
       & 50210.360      && I &&  0.027 & Peter H., 1996. \\
       & 50517.382      && I &&  0.024 & Peter H., 1997. \\
       & 50557.330      && I &&  0.029 & Peter H., 1997. \\
       & 50578.478      && II && 0.020 & Dedoch A., 1997. \\ 
       & 51281.9016     && II && 0.0276 & Diethelm R., 2001. \\
       & 51312.8758     && I &&  0.0285 & Diethelm R., 2001. \\
       & 51609.408      && I &&  0.029 & Paschke A., 2000. \\
       & 51676.7736     && I &&  0.0323 & Nelson R.H., 2001. \\}
}

\vspace*{-0.5cm}

\references

Aslan Z. \& Herczeg T.J., 1984, {\it IBVS}, No. 2478

Baldwin M.E., 1973, {\it IBVS} No. 795

Batten A.H., Fletcher J.M., McCarthy D.G., 1989, {\it Eight Catalogue of the Orbital Elements of Spectroscopic Binary Systems}, {\bf Vol. XVII}, p. 68 \& 214, Publ. of the Dominion Astrophysical Observatory.

Broglia P., 1960, {\it Contr. Oss. Milano-Merate}, Nuova Seria, {\bf N. 160}.

Dedoch A., 1995a, Contr. N. {\it Copernicus Obs. \& Plan. in Brno}, {\bf No. 31}, p. 20.

Dedoch A., 1995b, 1997 in {\it BBSAG} {\bf Nos. 109, 115}.

Derman E., Yilmaz Y., Engin S., Aslan Z., Aydin C., Tufekcioglu Z. 1982, {\it
IBVS}  No.~2159

Diethelm R., 1977, {\it Astron. Obs. Krakow} {\bf No. 48}.

Diethelm R., 2001, {\it IBVS} No. 5027

Germann R., 1982, 1983, 1986, 1987, 1988 in {\it BBSAG} {\bf Nos. 60, 67, 80, 83, 88}.

Kholopov P.N., 1985, {\it General Catalogue of Variable Stars}, {\bf Vol. I}, p. 300.

Koch R.H., 1961, {\it AJ} {\bf 66.}, 35.

Locher K., 1980, 1981, 1982, 1983, 1984 in {\it BBSAG} {\bf Nos. 46\&47, 52, 59, 65, 70}.  

McLean B.J. \& Hilditch R.W., 1983, {\it Mon.\ Not.\ Roy.\ Astr.\ Soc.}, {\bf 203}, 1.

Nelson R.H., 2001, {\it IBVS} No. 5040

Paschke A., 1994, 2000 in {\it BBSAG} {\bf Nos. 107, 123}.

Peter H., 1977, 1979, 1980, 1982, 1983, 1984, 1985, 1987, 1988, 1989, 1990,
1991, 1992, 1993, 1994, 1995, 1996, 1997 in {\it BBSAG} {\bf Nos.~33,~37\&43,~47\&48,~60,~65,~72\&73, 76, 84,~88,~91,~94\&95,~97\&98,~101,~104,~106\&107,~109,~112,~114~\&115}.

Rovithis, P. \& Rovithis-Livaniou, H., 1984, {\it A\&AS}, {\bf 58}, 679.

Struve O. \& Gratton L., 1948, {\it ApJ}, {\bf 108}, 497.

Wood F.B., Oliver J.P., Florkowski D.R. \& Koch R.H., 1980, {\it A Finding List for Observers of Interacting Binary Stars}, Un. of Pennsylvania Press.

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