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\IBVShead{4461}{21 March 1997}

\centerline{\ft UBV OBSERVATIONS OF T CrB}
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\begintext

T Coronae Borealis is a spectroscopic binary, with a period of 227.5 days,
containing an M3 giant and a hotter companion. The star has experienced two
outbursts with an amplitude of about 8 mag in 1866 and 1946, and is
classified as a recurrent nova as well as a symbiotic star. The ultraviolet
observations and the behaviour of the star during the outbursts
point to the hotter
component is an white dwarf with a mass close to the Chandrasekhar limit
(Selvelli, Cassatella and Gilmozzi, 1992), in spite of the existence of some doubts
that it can be a main sequence star (e.g. Kenyon and Garcia, 1986).
The M giant fills the Roche lobe
and the main part of the accretion is realized through the $L_{1}$.

    $UBV$ observations of the recurrent nova T CrB
were carried out with a single  channel  photon counting photometer,
mounted  at  the 0.6~m telescope of the  National  Astronomical
Observatory ``Rozhen". The comparison star was HD\,142929 $(V=8\fm 41$,
$B-V=0\fm 51$, $U-B=0\fm 03)$. The data was processed
using the APRN software (Kirov et al., 1991). The accuracy is better than $\pm 0.04$ mag.
Our observations are summarized in Table 1.

    The long term photometry in the $U$ band is presented in Figure 1. The triangles
indicate our data, the circles denote  the data from Hric et al. (1991, 1993, 1994) and
Skopal et al. (1992,1995), the crosses denote the data of Mikolajewski et al. (1996).
From  Figure 1 it is visible that since 1990 TCrB has experienced three
small outbursts with an amplitude of $\sim$1 mag in the $U$ band and peaks at
JD\,2448100, JD\,2449100 and JD\,2450200. The typical
time between these mini outbursts is of about 1000 days.
Since 1994 the star has shown a considerable increase in the $U$ brightness.
It is interesting to note that UV flux observations of
Selvelli, Cassatella and Gilmozzi (1992) over the period 1979-90
do not show similar behaviour. They had observed only two minima
in 1979 and 1989.

In the $V$ band as well as in the IR (see Yudin and Munari, 1993 and
references therein), TCrB shows a double wave light curve, as a result of
the ellipsoidal shape of the  M giant. In  Figure 2 the $V$ data are
shown, using the ephemeris of Kenyon and Garcia (1986).
The dots refer to the data obtained before 1989: Lines et al. (1988),
Rajkova and Antov (1986) and Bruch (1980, 1992).
The other symbols are the same as in Figure 1.
A Fourier analysis of the data using a three-term truncated Fourier
series yields

\medskip
$ V = (10.080\pm 0.005)+(0.018\pm 0.007)$cos$\phi+(0.162\pm 0.007)$cos$2\phi
     -(0.019\pm 0.007)$cos$4\phi$
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\hskip 1.0 cm
$ +(0.018\pm 0.008)$sin$\phi+(0.039 \pm 0.007)$sin$2\phi+(0.015\pm 0.008)$sin$4\phi, $
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\newline
where $\phi$ is the orbital phase. This fit is also plotted in Figure 2.
Although the data spread over 17 years
a distinction between the observations  obtained in
different epochs is not visible.  This points to the fact that the $V$ light curve
has not changed in its main features over the last 17 years.
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\centerline{2}
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\centerline{\epsffile{4461-f1.ps}}
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\centerline{Figure 1. $U$ band light curve of T CrB}

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\centerline{\epsffile{4461-f2.ps}}
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\centerline{Figure 2. Phase plot of the $V$ data according to} 
\centerline{the orbital  ephemeris of Kenyon and Garcia (1986)}
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   Most of the radiation flux in the $U$ is emitted from
the hotter component. The ellipsoidal variations are suggestive
of a large orbital inclination but an eclipse  cannot
be detected in the UV flux (Selvelli et al., 1992).
We also fail to detect eclipse  in the $U$ band.
The $U$ magnitudes of TCrB do not show correlation with the
orbital period. It is worth noting that the eccentricity
of the system is practically zero (Kenyon and Garcia, 1986) so the lack
of connection with the orbital period is not surprising.

   We ascribe the variability in the $U$ band to the accretion disk and/or the boundary layer between the disk and the white dwarf. In our opinion the most likely reason for the observed variability is the changes in the mass transfer rate and/or in the structure of the accretion disk.


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\centerline{3}
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\centerline{Table 1. Photometric observations of T CrB}
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\begin{center}
\begin{tabular}{|c|c|c|c|c|c|c|c|c|}
\hline
JD2400000+ & $V$ & $B-V$ & $U-B$& & JD  & $V$ & $B-V$ & $U-B$ \\
\hline
48321.44 & 10.40 & 1.40 &  0.37 & & 49889.34 &  10.19 & 1.45 &  0.51  \\
49046.57 & 10.11 & 1.54 &  0.92 & & 49929.37 &  10.16 & 1.36 &  0.32  \\
49046.60 & 10.08 & 1.54 &  0.84 & & 49930.39 &  10.17 & 1.33 &  0.18  \\
49047.54 & 10.09 & 1.51 &  0.68 & & 49994.23 &  10.16 & 1.36 &  0.34  \\
49104.53 & 10.24 & 1.24 &  0.12 & & 50092.66 &  10.08 & 1.43 &  0.57  \\
49106.45 & 10.25 & 1.18 & -0.05 & & 50141.64 &  10.23 & 1.08 & -0.20  \\
49195.32 & 10.25 & 1.61 &  1.21 & & 50142.57 &  10.29 & 1.27 &  0.08  \\
49197.32 & 10.26 & 1.56 &  1.11 & & 50167.52 &  10.19 & 1.46 &  0.62  \\
49452.51 & 10.40 & 1.53 &  0.80 & & 50192.39 &   9.90 & 1.14 &  0.13  \\
49520.40 & 10.01 & 1.53 &  1.30 & & 50267.36 &  10.03 & 1.16 & -0.22  \\
49572.32 & 10.44 & 1.56 &  0.92 & & 50268.36 &  10.01 & 1.14 & -0.30  \\
49573.29 & 10.44 & 1.52 &  0.78 & & 50297.30 &   9.88 & 1.22 &  0.08  \\
49597.27 & 10.23 & 1.49 &  0.61 & & 50393.18 &  10.19 & 1.25 &  0.05  \\
49598.29 & 10.21 & 1.49 &  0.64 & & 50433.66 &   9.84 & 1.29 &  0.11  \\
49882.33 & 10.16 & 1.37 &  0.38 & & 50435.66 &   9.87 & 1.29 &  0.12  \\
\hline
\end{tabular}
\end{center}

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This work was supported by NSF (F-466/94).

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\beginauthors
\authorsrightcol
R.K. ZAMANOV
NAO Rozhen, POB 136, 
4700 Smoljan, Bulgaria,
e-mail: rozhen@tu-plovdiv.bg 
V.I. ZAMANOVA
Planetarium, POB 132, 
4700 Smoljan, Bulgaria
\endauthors

\references

Bruch, A., 1980, {\it IBVS}, No.\,1805

Bruch, A., 1992, {\it A\&A}, {\bf 266}, 237

Kenyon, S.J., Garcia, M.R., 1986,  {\it AJ}, {\bf 91}, 125

Kirov, N. K., Antov, A.P., Genkov, V.V., 1991, {\it Compt. rend. Acad.
bulg. Sci.}, {\bf 44}, No.\,11, 5

Lines, H.C., Lines, R.D., McFaul, T.G., 1988,  {\it AJ}, {\bf 95}, 1505

Mikolajewski, M., Tomov, T., Kolev, D., 1996,  {\it IBVS}, No.\,4428

Hric, L., Skopal, A., Urban, Z. et al., 1991,
{\it Contr. Astr. Obs. Skalnat\'{e} Pleso}, {\bf 21}, 303

Hric, L., Skopal, A., Urban, Z., et al., 1993,
{\it Contr. Astr. Obs. Skalnat\'{e} Pleso}, {\bf 23}, 73

Hric, L., Skopal, A., Chochol, D., et al., 1994,
{\it Contr. Astr. Obs. Skalnat\'{e} Pleso}, {\bf 24}, 31

Rajkova, D., Antov, A., 1986, {\it IBVS}, No.\,2690

Selvelli, P.L., Cassatella, A., Gilmozzi, R., 1992, {\it ApJ}, {\bf 393}, 289

Skopal, A., Hric, L., Urban, Z., et al.,
1992, {\it Contr. Astr. Obs. Skalnat\'{e} Pleso}, {\bf 22}, 131


Skopal, A., Hric, L., Chochol, D., et al.,
1995, {\it Contr. Astr. Obs. Skalnat\'{e} Pleso}, {\bf 25}, 53

Yudin, B., Munari, U., 1993, {\it A\&A}, {\bf 270}, 165


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\title{CORRIGENDA}

\noindent
In the No.\,4428 issue of the IBVS, Figure 2 is erroneously the repetition
of Figure~1. The correct version of Figure 2 is as follows:
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\centerline{\epsffile{4428-f2.ps}}
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\normalsize
 Figure 2. T~CrB H$\alpha$ profiles in 1996. The equivalent width and
orbital phase are written in each box
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\large
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\centerline{$\star$$\star$$\star$}
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The hardcopy of IBVS No.\,4430 has been distributed in an incomplete form: the
last three references (page 4) are missing.
The references cut off the end of the paper are as follows:
\vskip 4mm

Krisciunas, K., Crowe, R.A., Luedeke, K.D., and Roberts, M., 1995,
{\it MNRAS}, {\bf 277}, 1404

Krisciunas, K., and Handler, G., 1995, {\it IBVS},  No.\,4195

Mantegazza, L., Poretti, E., and Zerbi, F.M., 1994, {\it MNRAS}, {\bf 270}, 439

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\noindent
The electronically accessible versions (both \LaTeX\ and PostScript) contain the
complete paper.

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\hspace*{11cm}The Editors

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