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\IBVShead{4437}{10 February 1997}

\centerline{\ft PHOTOELECTRIC {\large $BVI_c$} OBSERVATIONS, NEW ELEMENTS}
\centerline{\ft AND A NEW CLASSIFICATION FOR BZ Tuc}
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\begintext

BZ Tuc = HV 821 was included in our program of photoelectric observations
for Cepheids because it is listed in GCVS-IV as a classical
Cepheid with the elements
    $$Max JD = 2444141.64 + 127.61 \times E.$$
We observed the star at CTIO during September-November 1996 using
the 1.0--m reflector.  A total of 26 $BVI_c$ measurements were obtained
(Table 1), the accuracy of the individual data being near $\pm 0\fm 01$
in all filters. Our new observations are plotted as filled dots in Figure 1,
while open circles refer to our earlier observations (Berdnikov \& Turner, 1995).

The slight offset of the new observations from our earlier observations in
Figure 1 suggests that our data do not satisfy the above elements. In order
to refine them, we analyzed all available published observations using
Hertzsprung's method; the derived epochs of maxima, listed in Table 2,
together with times of maxima from Leavitt (1908), were introduced
into a linear least squares solution to obtain the following
improved ephemeris:
   $$Max JD = 2430242.8 + 127.447 \times E.$$
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\centerline{Table 1}
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\begin{center}
\begin{tabular}{cccccccc}
\hline
$JD$&$V$&$B-V$&${V-I}_c$&$JD$&$V$&$B-V$&${V-I}_c$\\
2450300+ & & & &2450300+ & & & \\
\hline
51.6601&11.794& 0.793& 0.874&   81.6460&11.680&  0.932&  0.954\\
52.7280&11.763& 0.800& 0.867&   82.6389&11.661&  0.940&  0.952\\
53.6278&11.756& 0.808& 0.867&   83.6399&11.650&  0.943&  0.966\\
54.6378&11.748& 0.807& 0.871&   84.6350&11.709&  0.969&  0.985\\
55.6658&11.784& 0.799& 0.872&   86.6257&11.691&  0.981&  0.969\\
57.6306&11.735& 0.804& 0.866&   87.6555&11.712&  0.998&  0.976\\
58.6354&11.738& 0.811& 0.867&   88.6383&11.719& 1.009& 1.005\\
59.6230&11.719& 0.824& 0.869&   89.6342&11.743& 1.023& 1.020\\
61.6585&11.700& 0.823& 0.865&   90.6310&11.750& 1.051& 1.014\\
62.6531&11.704& 0.836& 0.875&   91.6228&11.771& 1.055& 1.036\\
63.6294&11.706& 0.820& 0.889&   92.6297&11.779& 1.072& 1.036\\
79.6424&11.652& 0.893& 0.955&   93.6331&11.800& 1.073& 1.046\\
80.6372&11.669& 0.948& 0.954&   94.6770&11.810& 1.097& 1.051\\
\hline
\end{tabular}
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\centerline{2}
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\centerline{Table 2}
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\begin{center}
\begin{tabular}{ccccrcc}
\hline
$Max JD$ & Uncertainty & Filter & $E$ & $O-C$\ & Number of & Author \\
2400000+ & & & & & Observations & \\
\hline
\\
10097.00&     --  &pg &$-$158 & $-$9.17 & -- & Leavitt, 1908 \\
34443.23&$\pm$1.15 & B &  33 & $-$5.32 &10 & Gascoigne, Kron, 1965\\
34449.94&$\pm$1.91 & V &  33 &  1.39 &12 & Gascoigne, Kron, 1965\\
41070.64&$\pm$0.33 & B &  85 & $-$5.15 &18 & Eggen, 1977         \\
41203.26&$\pm$0.64 & V &  86 &  0.02 &25 & van Genderen, 1983  \\
41203.86&$\pm$0.48 & V &  86 &  0.63 &18 & Eggen, 1977         \\
41325.48&$\pm$0.32 & B &  87 & $-$5.21 &23 & van Genderen, 1983  \\
41834.12&$\pm$0.40 & B &  91 & $-$6.35 &10 & Madore, 1975        \\
41839.31&$\pm$1.11 & V &  91 & $-$1.17 &10 & Madore, 1975        \\
44011.44&$\pm$1.17 & V & 108 &  4.37 & 9 & Freedman at el., 1985\\
44135.92&$\pm$0.85 & V & 109 &  1.40 & 6 & Harris, 1980         \\
44515.26&$\pm$0.29 & B & 112 & $-$1.60 &34 & Caldwell, Coulson, 1984\\
44521.02&$\pm$0.34 & V & 112 &  4.16 &37 & Caldwell, Coulson, 1984\\
50117.60&$\pm$0.31 & V & 156 & $-$6.92 &32 & This paper\\
50238.23&$\pm$0.24 & B & 157 & $-$13.74 &30 & This paper\\
\hline
\end{tabular}
\end{center}

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Figure 1.  The light curve of BZ Tuc established by our earlier observations
(Berdnikov \& Turner 1995), open circles, and the observations of Table 1,
filled circles
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Figure 2.  The O$-$C diagram for BZ Tuc.  For convenience the O$-$C values are
expressed in fractions of the period
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\centerline{3}
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Figure 3.  The light curve of BZ Tuc according to van Genderen (1983), top,
Caldwell \& Coulson (1984), middle, and Eggen (1977), bottom
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The new ephemeris was used to calculate the $O-C$ values listed in Table 2,
as well as for plotting Figures 2 and 3. In both Table 2 and Figure 2
we have taken into account that maxima in filter $B$ precede those in
$V$ by 6.0 days.  The data of Figure 1, as well as the observations of
van Genderen (1983), Caldwell \& Coulson (1984), and Eggen (1977),
which are replotted in Figure 3a--c for the new ephemeris, indicate
that the shape of the light curve of BZ Tuc varies slightly.  Moreover, a
shift in the times of maxima for $B-V$ relative to those in $V$ is
evident.  Such variability in light curve shape suggests that BZ Tuc
cannot be a classical Cepheid.  Likewise, it cannot be a type II Cepheid
because of its very long period.  Possibly BZ Tuc is an RV Tauri variable
or alternatively a semiregular variable of the UU Herculis class.
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\centerline{4}
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The research described here was made possible in part by grants
No. 95--02--05276 from the Russian
Foundation of Basic Research to LNB and through NSERC Canada to
DGT.  The authors were Visiting Astronomers at Cerro Tololo
Inter-American Observatory, National Optical Astronomy
Observatories, which is operated by the Association of
Universities for Research in Astronomy, Inc. (AURA) under
co-operative agreement with the National Science Foundation.
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\beginauthors
 L.N.~BERDNIKOV
 Sternberg Astronomical Institute
 13, Universitetskij prosp.
 Moscow 119899, Russia
\authorsrightcol
 D.G.~TURNER
 Saint Mary's University
 Halifax, Nova Scotia, B3H 3C3
 Canada
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\endauthors

\references

Berdnikov, L.N., \& Turner D.G., 1995, {\it Astron. Letters}, {\bf 21}, 534

Caldwell, J.A.R., \& Coulson, I.M., 1984, {\it South. Afr. Astron. Observ. Circ.},
        No.\,8, 1

Eggen, O.J., 1977, {\it Astrophys. J. Suppl.}, {\bf 34}, 33

Freedman, W.L., Grieve, G.R., \& Madore, B.F., 1985,
       {\it Astrophys. J. Suppl.}, {\bf 59}, 311

Gascoigne, S.C.B., \& Kron G.E., 1965, {\it M.N.R.A.S.}, {\bf 130}, 333

Harris, H.C., 1980, Ph.D. Thesis, University of Washington

Leavitt, H.C., 1908, {\it Ann. Astron. Observ. Harvard College},  {\bf 60}, 1

Madore, B.F., 1975, {\it Astrophys. J. Suppl.}, {\bf 29}, 219

van Genderen, A.M., 1983, {\it Astron.  Astrophys. Suppl.}, {\bf 52}, 423

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