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\IBVShead{5323}{21 October 2002}

\IBVStitle{CCD Photometry of T UM\lowercase{i}}

\IBVSauth{\v{S}melcer, L.}

\IBVSinsto{Vala\v{s}sk\'{e} Mezi\v{r}\'{\i}\v{c}\'{\i} Observatory,
Vset\'{\i}nsk\'{a} 78, 757 01 Vala\v{s}sk\'{e}
Mezi\v{r}\'{\i}\v{c}\'{\i}, Czech Republic,\\  e-mail: {\tt lsmelcer@astrovm.cz}}

\SIMBADobj{T UMi}
\IBVStyp{  M  }
\IBVSkey{mirae - photometry}


\begintext

T Ursae Minoris (= GSC 04408 00163 = HD 118556; 
$\alpha = 13\hr 34\mm 41.09\sec$, $\delta = 73\deg 25\arcm 53.10\arcs$ [J2000])
is a Mira type star, with spectral type varying between M4e and M6e and with 
range of optical light variations between visual magnitudes 7.8 and 15.0.
In the fourth edition of the GCVS (Kholopov et al., 1985) a period of 301 days 
is given for its light variations. Detailed analysis of its light curve came 
out in papers of G\'al and Szatm\'ary (1995)
and Mattei and Foster (1995). G\'al and Szatm\'ary (1995) showed that the period of T~UMi is strongly 
decreasing: between JD 2440000 and JD 2449250 the period dropped from 314.5 days to 283.2 days. 
They attributed this change to a change of luminosity due to a shell flash in the helium burning 
shell. The period was constant before JD 2440000 suggesting that T~UMi is just after the beginning 
of the shell flash. Using calculations of Wood and Zarro (1981), G\'al and Szatm\'ary (1995) suggested that the 
period will start increasing again in few decades.

CCD photometry of T~UMi has been done at Vala\v{s}sk\'{e}
Mezi\v{r}\'{\i}\v{c}\'{\i} observatory between 1999 January and
2001 May using astrocamera ZEISS 120/540 mm with attached SBIG
ST-7 CCD camera and $V$ filter. CCDOPS software bundled with SBIG
cameras was used for photometry. GSC 4408 01074 (= PPM 8412 = SAO
7813 = BD +74 540; $V$ = 9.28 mag, $B-V$ = 0.93 mag) was used as
the comparison star. A total of 456 measurements has been obtained
in the course of 4 cycle. Measurements have typical errors of about 0.03 mag. 
Four maxima timings could be determined using the Kwee and von Woerden (1956) 
method implemented in AVE (Barbera, 2000) and their values are given in Table 1.
The light curve of T~UMi is shown in Figure 1. Observations can be retrieved
through IBVS website (5323-t2.txt).

\IBVSedata{5323-t2.txt}
%\IBVSdataKey{5323-t2.txt}{T UMi}{photometry}

An $O-C$ diagram of maxima timings of T~UMi based on data from AFOEV database and observations
from Table 1 is presented in Figure 2.
Changes of the length of the period (distances of subsequent maxima) are plotted in Figure 3.
The fitted line corresponds to the period decrease of 2.3 days/cycle.
The period used for construction of Figure 2 was
taken from GCVS (301 d) and JD 2443052 was used as the basic maximum.


\begin{table}[h]
\caption{Maxima timings of T~UMi.}
\begin{center}
\begin{tabular}{lcccclll}
\hline
Geo.\ JD & Error & Filter & Maxima & $O-C$ &\\
\hline
2451267.8 & 0.5  &  $V$    &  30   &$-$513 &\\
2451494.5 & 1.3  &  $V$    &  31   &$-$587 &\\
2451738.4 & 0.3  &  $V$    &  32   &$-$645 &\\
2451989.5 & 0.2  &  $V$    &  33   &$-$695 &\\
\hline
\end{tabular}
\end{center}
\end{table}

{\it Acknowledgements:} This work has made use of the SIMBAD
database, operated at CDS, Strasbourg, France. The NASA ADS
Abstract Service was used to access data and references.

The author is thankful to Ond\v{r}ej Pejcha for help with
preparation of figures.


\IBVSfig{7cm}{5323-f1.eps}{Light curve of T UMi.}
%\IBVSfigKey{5323-f1.eps}{T UMi}{light curve}

\IBVSfig{7cm}{5323-f2.eps}{$O-C$ diagram of maxima timings of
T~UMi based on data from the AFOEV database (maxima 1-34; open
squares) and from observations from this paper (maxima 30-33; solid
circles). The solid curve is a quadratic fit to the data.}
%\IBVSfigKey{5323-f2.eps}{T UMi}{O-C diagram}

\IBVSfig{7cm}{5323-f3.eps}{Evolution of the length of the period. Symbols are same as in Figure 2. 
The solid line corresponds to period decrease of 2.3 days/cycle.}
%\IBVSfigKey{5323-f3.eps}{T UMi}{}

\references

Barbera, R., 2000, {\tt http://www.astrogea.org/soft/ave/aveint.htm} \strut

G\'al, J., Szatm\'ary, K., 1995, {\it A\&A}, {\bf 297}, 461

Kholopov, P. N. et al., 1985, {\it General Catalogue of Variable Stars}, 4th edition, Moscow

Kwee, K. K. and Van Woerden, H., 1956, {\it BAN}, {\bf 12}, No.\,464, 327

Mattei, J.A., Foster, G., 1995, {\it JAAVSO}, {\bf Vol.23}

Wood, P.R., Zarro, D.M., 1981, {\it ApJ}, {\bf 247}, 247

\endreferences

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