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\IBVShead{5188}{19 October 2001}

\IBVStitletl{Further Improvement of the Period}{ and New R Light Curve of CQ UM\lowercase{a}}

\IBVSauth{Mikul\'{a}\v{s}ek, Z.$^{1,2}$; \v{Z}i\v{z}\v{n}ovsk\'{y}, J.$^{3}$}

\IBVSinst{Institute of Theoretical Physics and Astrophysics, Masaryk
University in Brno, Kotl\'{a}\v{r}sk\'{a} 2,\\ \indent 611 37 Brno,
Czech Republic, e-mail: mikulas@ics.muni.cz}

\IBVSinst{Nicolaus Copernicus Observatory and Planetarium, Krav\'{\i}
Hora 2, 616 00 Brno, Czech Republic}

\IBVSinst{Astronomical Institute, Slovak Academy of Sciences, 059 60
Tatransk\'{a} Lomnica, Slovak Republic\\ \indent e-mail: ziga@ta3.sk}

\SIMBADobj{CQ UMa}

\IBVSkey{photometry}

\IBVSabs{We report new period improvement for CQ UMa based on new}
\IBVSabs{photoelectric observations. A new light-curve in the red} 
\IBVSabs{region is published.}

\begintext

The cool chemically peculiar SrCrEu star CQ UMa = HR 5153 = HD 119213 ($m_V = 6.28$)
belongs to the best photometrically monitored stars of its type. The star 
exhibits relatively large light variations in the blue (namely in 
Str\"{o}mgren's $v$ colour) and smaller antiphased ones in the red. While 
in the blue and yellow regions hundreds of reliable photometric 
measurements exist, our knowledge about the light behaviour of the star in 
the red and near infrared only reposed on several measurements done by 
Musielok et al.\ (1980) as a part of their intermediate band ten-colour 
photometry.

The requirement of reliable red data and need for enlargement of the
time base of light variation measurements essential for further
improvement of the period of the star induced us to start with
systematic observations of CQ UMa in Str\"{o}mgren's $v$ filter and
Johnson's $R$ filter.

The detailed history of the CQ UMa period determination is published in
Mikul\'{a}\v{s}ek (1987). The latest improvement of light elements
based on 215 $B$ and 102 $v$ measurements referred
to the more or less symmetrical minimum of light in $v$ colour were
published by \v{Z}i\v{z}\v{n}ovsk\'{y} \& Mikul\'{a}\v{s}ek (1995):
$${\rm JD_{hel}(Min}~v) = 2 445 349.7263(47) + (E - 1878)\times
2\fday4499141(38).$$

In this paper we present 54 measurements in $R$ and 30 new measurements
in $v$ taken in 56 individual moments in the time interval from March
1994 to May 2000. All photometric measurements were done by the red
sensitive photometer attached to the 0.6-m telescope of the
Skalnat\'{e} Pleso Observatory.  HD 120874 = HR 5216 ($m_V = 6.46$) was
used as a comparison star.  All new data obtained were used for the
improvement of the period of CQ UMa.

The comprehensive examination of all currently available photometric data 
(including our new data --- see Figs.~1 and~2) particularly confirmed that each 
of the observed light curves in the region at least 350 nm to 800 nm can be well 
enough represented by the linear combination of a constant and two 
basic harmonic polynomials of the second order (Mikul\'{a}\v{s}ek, 1994). 
Hence we could apply our newly developed method for an improvement of period 
of periodically variable stars (Mikul\'{a}\v{s}ek, in preparation) to all 
accessible photometric data with sufficient amplitude of variations/noise 
ratio. 
In the total we have used 884 measurements of nine authors (Burke \& Howard, 
1972; ESA, 1997; Jetsu et al., 1992; Mikul\'{a}\v{s}ek et al., 1978; 
Musielok et al., 1980; Pavlovski, 1979; Pyper \& Adelman, 1985; Winzer, 1974; 
Wolff \& Morrison, 1975; this paper) obtained in the $u, v, b, U, B, R$ 
and $H$ colours, the last being the instrumental colour of the Hipparcos 
satellite. The whole material more or less uniformly covers the time interval 
of thirty years or 4457 stellar revolutions.

\IBVSfig{9cm}{5188-f1.eps}{The $v$ light curve of CQ UMa. Smooth line:
the fitted light curve. Symbols:  $\circ$ Pyper \& Adelman (1985),
$\times$ Musielok et al.\ (1980), + Wolff \& Morrison (1975), $\bullet$
this paper}

\IBVSfig{3.38cm}{5188-f2.eps}{The $R$ light curve of CQ UMa. Dots:
observations, smooth line: the fitted light curve}

The times of $v$ minima are given by the relation:
$${\rm JD_{hel}(Min}~v) = 2 445 925.4255(37) + (E - 2113)\times
2\fday4499117(29),$$
the initial epoch ($E = 0$) corresponds to the $v$ colour light minimum
immediately preceding the first photometric observation of CQ UMa. 
The reliability of this ephemeris is extreme, the standard uncertainty 
of the phase determination being 0.002. 

Our new photometry indicates that the period of light variations of the star 
was stable within the last 30 years, which confirms an incredible stability 
of photometric patterns on the stellar surface responsible for the light 
variability.

\bigskip

This work was supported by VEGA grant No.~7107.

\references

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ESA, 1997, in {\it The Hipparcos and Tycho Catalogues}, ESA SP-1200

Jetsu, L., Kokko, M., Tuominen, I., 1992, {\it A\&A}, {\bf 256}, 547
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Pyper, D. M., Adelman, S. J., 1985, {\it A\&AS}, {\bf 59}, 369
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\v{Z}i\v{z}\v{n}ovsk\'{y}, J., Mikul\'{a}\v{s}ek, Z., 1995, {\it IBVS}, No.\ 4259
%\BIBCODE{1995IBVS.4259....1Z}

\endreferences
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