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\IBVShead{4259}{24 October 1995}

\centerline{\ft IMPROVEMENT OF THE PERIOD OF CQ UMa}
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\begintext

CQ UMa (HD\,119 213, HR 5153) is a cool CP2 star demonstrating strong
periodic light and spectrum variations caused by the presence of large
spectroscopic and photometric spots on the surface of the rotating
star. The first correct value of the period: P = 2.451 days, was found
by Wolff $\&$ Morrison (1975) on the basis of their Str\"{o}mgren $uvby$
photometry. Shapes and amplitudes of light curves are
wavelength-dependent. The largest variations (about 0.1 mag) take
place in the $v$ color. Longwards of 520 nm light variations are in the
antiphase with variations in the blue region exceeding the amplitude
of 0.02 mag.

Modelling of photometric spots on the stellar surface requires very
good knowledge of light curves in various colors (Mikul\'{a}\v{s}ek, 1994). As
the published measurements of the star brightness taken by many
authors were obtained within the time interval of 25 years, their
exploitation is possible only if the period of the star is known
with satisfying accuracy.

In the search for the precise period of CQ UMa we have used all the
available photometric observations in Johnson's B, Str\"{o}mgren's $v$ and
the Shemakha Observatory's X filters, in which the S/N ratio is the
best. The last of the filters is very similar to that of Str\"{o}mgren's $v$ 
filter
(Sch\"{o}neich $\&$ Staude, (1976), Musielok et al. (1980)). Nine sets of 
photometry used, 
cover 3720 revolutions of the star -- see the list in the Table. The last
set of photometry taken in the $v$ filter was recently obtained by the
team involving one of us (J. \v{Z}.) by means of the photoelectric 
photometer 
attached to the 0.6 m telescope of the Skalnat\'{e} Pleso Observatory.

\begin{table}[h]
\begin{tabular}{lcrccc}
\hline
\hline
Source & col & N & cycles & Mean Epoch & (O--C)$_{\rm new}$\\
\hline
Burke $\&$ Howard (1972) & B & 26 & 0 -- 158 & 84 & +0.007 $\pm$ 0.036\\
Winzer (1974) & B & 18 & 145 -- 291 & 198 & +0.006 $\pm$ 0.016\\
Wolff $\&$ Morrison (1975) & $v$ & 25 & 401 -- 443 & 427 & --0.007 $\pm$ 0.015\\
Musielok et al. (1980) & X & 29 & 604 -- 894 & 761 & +0.003 $\pm$ 0.008\\
Mikul\'{a}\v{s}ek et al. (1978)\\
+ Pavlovski (1979) & B & 28 & 441 -- 1215 & 969 & +0.011 $\pm$ 0.017\\
Pyper $\&$ Adelman (1985) & $v$ & 24 & 1465 -- 2057 & 1834 & --0.005 $\pm$ 0.015\\
Jetsu et al. (1991) & B & 143 & 2954 -- 3138 & 3029 & --0.001 $\pm$ 0.007\\
This paper & $v$ & 24 & 3638 -- 3720 & 3670 & +0.012 $\pm$ 0.018\\
\hline
\hline
\end{tabular}
\end{table}
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The period improvement has been carried out using special iterative
least squares method (details in Mikul\'{a}\v{s}ek et al., 1995). The basic
assumptions of the
analysis of data were: the light curves in B and $v$ colors are constant
(but generally unequal) and the periods of these variations are the same.
On the contrary to the previous papers dealing\break

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\begin{center}
Figure 1.
Light curve of CQ UMa in X and $v$ filters. Circles  -- data from
literature, crosses -- this paper, brightness in millimagnitudes relative to the mean value.
Phases were calculated according to the ephemeris given in this paper.\end{center}
\large
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\noindent with the CQ UMa light
elements, we put the beginning of counting of cycles at the light
minimum of the $v$ color. The minimum of the $v$ light curve is sharp,
symmetric and so well defined, while the maximum in both  colors
used before is both flat and asymmetric.

Altogether 215 measurements in B and 102 in $v$ colors were used and the
following ephemeris has been derived:

\begin{center}
JD$_{\rm hel}$(Min $v$) = 2 445 349.7263 + (E--1878) 2\fd 4499141 \\
%\end{center}
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~\ $\pm$0.0047~~~~~~~~~~~~~\ \,$\pm$0.0000038
\end{center}
where the epoch E was chosen so, that E = 0 corresponds to the $v$ light
minimum immediately preceding the first of published photometric
observations of CQ UMa.


(O--C) values listed in the Table indicate that the period of the
variations of the star was stable within the last 25 years, what
confirms the expected extreme stability of photometric patterns on the
stellar surface. Accuracy of the period determination enables one to
find the phase of observations during the last 25 years with an
uncertainty not exceeding 0.003, what is the basic requirement for
further light curve analysis and mapping of the star's atmosphere.
The phase $\phi$ can be computed according to the following relation:


\begin{center}
$\phi$ = frac[(JD$_{\rm hel}$ -- 2 440 748.7876)/2.4499141]
\end{center}
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\centerline{3}
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The observed $v$ light curve minimum is at the phase $\phi$ = 0.000$\pm$0.007,
but in the B one finds the minimum at $\phi$ = --0.021$\pm$0.006(!). On
the contrary, the flat maxima of light curves occur at the same
phases: 0.581$\pm$0.011 and 0.574$\pm$0.023 in $v$ and B colors, respectively
(see  Figure 1). Detailed analysis of light curves will be published
elsewhere. Photometric observations from Skalnat\'{e} Pleso Observatory
would be at your disposal via e-mail: ziga@ta3.sk.\\

This work was supported, in part, by Grant Agency for Science (grant No. 
2/62/95).
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\beginauthors
Jozef \v{Z}I\v{Z}\v{N}OVSK\'{Y}
Astronomical Institute
Slovak Academy of Sciences
059 60 Tatransk\'{a} Lomnica
Slovak Republic
e-mail: ziga@ta3.sk
\vskip 2mm
\authorsrightcol
Zden\v{e}k MIKUL\'A\v{S}EK
N. Copernicus Observatory
and Planetarium
Krav\'{\i} hora 2
616 00 Brno
Czech Republic
e-mail: mikulas@vm.ics.muni.cz
\endauthors

\references

Burke, E. W., Howard, J. T., 1972, {\it Astrophys. J.}, {\bf 178}, 491

Jetsu, L., Kokko, M., Tuominen, I., 1991, private communication

Mikul\'{a}\v{s}ek, Z., Harmanec, P., Grygar, J., \v{Z}\d\'{a}rsk\'{y}, F., 
1978, {\it Bull. Astron. Inst. Czechosl.}, {\bf 29}, 44

Mikul\'{a}\v{s}ek, Z., 1994, in: Chemically Peculiar and Magnetic Stars,
J. Zverko and J. \v{Z}i\v{z}\v{n}ovsk\'{y} (eds.), Astronomical Institute, 
Slovak Academy of Sciences, Tatransk\'{a} Lomnica,  p.\,165

Mikul\'{a}\v{s}ek, Z., Han\v{z}l, D., Hroch, F., Hornoch, K., 1995, {\it Contr. N. Copernicus Obs. Plan.}, No.\,31 (in press)

Musielok, B., Lange, D., Sch\"{o}neich W., Hildebrandt G., \v{Z}elwanova E.,
Hempelmann, A., 1980, {\it Astron. Nachr.}, {\bf 301}, 71

Pavlovski, K., 1979, {\it Astron. Astrophys.}, {\bf 76}, 362

Pyper, D. M., Adelman, S. J., 1985, {\it Astron. Astrophys. Suppl. Ser.}, 
{\bf 59}, 369

Sch\"{o}neich, W., Staude, J., 1976, in: Physics of Ap Stars, W. W. Weiss,
H. Jenkner and H. J. Wood (eds.), Universit\"{a}tssternwarte, Wien, p. 387

Winzer, J. E., 1974, unpubl. Thesis, Univ. of Toronto

Wolff, S. C., Morrison, N. D., 1975, {\it Publ. Astron. Soc. Pac.}, {\bf 87}, 231

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