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\IBVShead{4389}{1 November 1996}

\centerline{\ft PHOTOELECTRIC BVI$_C$ OBSERVATIONS AND NEW ELEMENTS}
\centerline{\ft FOR THE RR LYRAE STAR SU Col}
\vskip 8mm

\begintext

SU Col was included in our program of photoelectric observations
for Cepheids because it is classified in GCVS-IV as a type II
Cepheid with a period of P = 21.55 days.  We observed the star at
CTIO in September and October 1996 using the 0.6--m reflector.
A total of 75 $BVI_c$ measurements were obtained (Table 1), the
accuracy of the individual data being near $\pm$0\fm 01 in all
filters.

As observations were accumulated it became clear that SU Col
is not a Cepheid.  Gessner (1985) suggested previously
that SU Col is an RR Lyrae star with the elements:
\vskip 1mm
\centerline{Max JD$_{hel}$ = 2428763.655 + 0.487361 $\times$ E.}
\vskip 1mm
Those elements are used in Figure 1 for plotting our new
observations.  The data indicate that SU Col is indeed an
RR Lyrae variable and that the amplitude of the light 
curve is at present 0\fm 77 in $V$, 0\fm 29 in $B-V$ and
0\fm 33 in ${V-I}_c$, while the epoch of maximum light is
JD\,2450363.870 $\pm 0.005$.

\epsfysize=10cm
\centerline{\epsffile{4389-f1.ps}}
\centerline{Figure 1}
\vfill \eject

\centerline{2}
\vskip 1mm
\centerline{Table 1}
\vskip 1mm
\begin{center}
\begin{tabular}{cccccccc}
\hline
$JD_{hel}$&$V$&$B-V$&${V-I}_c$&$JD_{hel}$&$V$&$B-V$&${V-I}_c$\\
2450300+ & & & &2450300+ & & & \\
\hline
\\
51.7380&  12.266& 0.249& 0.350&  61.7894&  12.859& 0.399& 0.562\\
52.7967&  12.493& 0.352& 0.472&  61.8065&  12.864& 0.427& 0.562\\
53.6872&  12.268& 0.255& 0.356&  61.8341&  12.878& 0.393& 0.564\\
54.7276&  12.446& 0.335& 0.444&  61.8595&  12.807& 0.357& 0.536\\
54.7705&  12.588& 0.358& 0.529&  61.8785&  12.647& 0.319& 0.449\\
55.7282&  12.573& 0.347& 0.501&  62.6872&  12.795& 0.403& 0.574\\
55.7856&  12.711& 0.358& 0.574&  62.7051&  12.806& 0.425& 0.568\\
57.6904&  12.570& 0.361& 0.495&  62.7201&  12.812& 0.418& 0.581\\
57.7278&  12.647& 0.352& 0.560&  62.7278&  12.830& 0.401& 0.570\\
57.7492&  12.683& 0.377& 0.571&  62.7408&  12.837& 0.402& 0.567\\
57.7693&  12.702& 0.414& 0.567&  62.7478&  12.822& 0.402& 0.560\\
57.7805&  12.714& 0.411& 0.547&  62.7613&  12.840& 0.416& 0.560\\
57.7899&  12.730& 0.409& 0.566&  62.7743&  12.878& 0.414& 0.554\\
57.8005&  12.761& 0.398& 0.576&  62.7838&  12.876& 0.436& 0.544\\
57.8131&  12.763& 0.398& 0.564&  62.7947&  12.889& 0.426& 0.562\\
57.8435&  12.766& 0.404& 0.561&  62.8109&  12.888& 0.398& 0.551\\
57.8590&  12.786& 0.403& 0.566&  62.8205&  12.866& 0.398& 0.545\\
58.6772&  12.615& 0.359& 0.535&  62.8323&  12.827& 0.389& 0.519\\
58.7077&  12.676& 0.384& 0.552&  62.8428&  12.718& 0.347& 0.496\\
58.7475&  12.714& 0.382& 0.575&  62.8535&  12.668& 0.300& 0.490\\
58.7524&  12.717& 0.412& 0.579&  62.8618&  12.577& 0.301& 0.425\\
58.7888&  12.748& 0.401& 0.587&  62.8775&  12.372& 0.237& 0.365\\
58.8315&  12.790& 0.393& 0.575&  63.6884&  12.819& 0.452& 0.578\\
58.8659&  12.818& 0.418& 0.553&  63.7045&  12.835& 0.422& 0.570\\
59.6831&  12.667& 0.383& 0.544&  63.7256&  12.856& 0.410& 0.556\\
59.7200&  12.719& 0.397& 0.584&  63.7381&  12.880& 0.411& 0.560\\
59.7409&  12.749& 0.397& 0.564&  63.7443&  12.881& 0.415& 0.582\\
59.7685&  12.766& 0.401& 0.557&  63.7564&  12.900& 0.404& 0.559\\
59.7736&  12.777& 0.400& 0.580&  63.7717&  12.920& 0.389& 0.580\\
59.8069&  12.810& 0.390& 0.572&  63.7809&  12.906& 0.408& 0.556\\
59.8541&  12.854& 0.405& 0.573&  63.8017&  12.859& 0.390& 0.545\\
59.8696&  12.877& 0.395& 0.569&  63.8181&  12.721& 0.333& 0.490\\
59.8806&  12.864& 0.403& 0.587&  63.8287&  12.618& 0.326& 0.433\\
61.6903&  12.757& 0.395& 0.581&  63.8379&  12.548& 0.292& 0.408\\
61.7222&  12.768& 0.416& 0.559&  63.8484&  12.423& 0.259& 0.391\\
61.7362&  12.816& 0.394& 0.577&  63.8588&  12.257& 0.218& 0.319\\
61.7631&  12.814& 0.414& 0.552&  63.8752&  12.140& 0.157& 0.288\\
61.7792&  12.842& 0.425& 0.557&         &        &     &     \\
\hline
\end{tabular}
\end{center}
\vfill \eject

\centerline{3}
\vskip 1mm
\begin{center}
\begin{tabular}{ccrr}
\hline
$Max JD_{hel}$ & Uncertainty & $E$ & O$-$C \\
2400000+ & & & \\
\hline
\\
28763.659&  $\pm0.01$&   $-$44321&    0.001\\
28783.636&  $\pm0.03$&   $-$44280&   $-$0.003\\
28785.643&  $\pm0.03$&   $-$44276&    0.054\\
28787.597&  $\pm0.03$&   $-$44272&    0.059\\
28848.458&  $\pm0.03$&   $-$44147&    0.000\\
28849.519&  $\pm0.03$&   $-$44145&    0.086\\
28874.342&  $\pm0.03$&   $-$44094&    0.054\\
28891.357&  $\pm0.03$&   $-$44059&    0.012\\
28893.327&  $\pm0.03$&   $-$44055&    0.032\\
28930.325&  $\pm0.01$&   $-$43979&   $-$0.009\\
28933.314&  $\pm0.03$&   $-$43973&    0.056\\
34303.488&  $\pm0.03$&   $-$32954&    0.027\\
34325.437&  $\pm0.03$&   $-$32909&    0.045\\
38374.403&  $\pm0.03$&   $-$24601&    0.037\\
38377.379&  $\pm0.03$&   $-$24595&    0.089\\
38697.521&  $\pm0.03$&   $-$23938&    0.037\\
38785.291&  $\pm0.03$&   $-$23758&    0.082\\
39150.286&  $\pm0.03$&   $-$23009&    0.046\\
39409.570&  $\pm0.03$&   $-$22477&    0.055\\
39410.558&  $\pm0.03$&   $-$22475&    0.068\\
50363.870& $\pm0.05$&        0&    0.000\\
\hline
\end{tabular}
\end{center}
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\centerline{\epsffile{4389-f2.ps}}
\centerline{Figure 2}
\vfill \eject

\centerline{4}
\vskip 1mm

The slight offset in phase of the light curve in Figure 1 implies
that Gessner's elements do not satisfy our observations.  In
order to derive a more reliable period, we searched Gessner's
data for times near maximum photographic brightness and combined
the epochs with the present results.  Two epochs taken from
Gessner's data appear to lie very close to true light maximum
since the star was brighter at those times than at others.
Those epochs were assigned an uncertainty of $\pm$0\fd 01,
while the remaining 20 instants near maximum photographic
brightness published by Gessner (1985) were assigned an
uncertainty of $\pm$0\fd 03 (Table 2).  The latter appear to
be displaced slightly towards the declining light portion of the
light curve, and result in the somewhat skewed distribution of
data points in the O$-$C diagram (Figure 2).  A linear least
squares analysis of the resulting O$-$C data (with weights
inversely proportional to the squares of the associated
uncertainties) gave the following improved ephemeris:
\vskip 1mm
\centerline{Max JD$_{hel}$ = 2450363.870 + 0.48735842 $\times$ E.}
\hspace{66mm} $\pm 0.002\ \ \ \pm$ 0.00000011
\vskip 1mm
The new elements were used to calculate the O$-$C values listed in
Table 2.

The research described here was made possible in part by grants
No. 95--02--05276 and No. 94--02--04347 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.

\vskip 1cm

\beginauthors
\authorsrightcol
 L.N.~BERDNIKOV
 Sternberg Astronomical Institute
 13, Universitetskij prosp.
 Moscow 119899, Russia
\vskip 2mm
 D.G.~TURNER
 Saint Mary's University
 Halifax, Nova Scotia, B3H 3C3
 Canada
\endauthors

%\references

Reference:

Gessner, H., 1985, {\it Mitt. Ver\"anderl. Sterne Sonneberg}, {\bf 10}, 155


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