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\IBVShead{4203}{6 June 1995}

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\begin{center}
{\ft UBV OBSERVATIONS OF AB Dor, 1994-5}
\end{center}

\vskip .8cm
\begintext

The active, cool (early K type) dwarf  AB Dor (= HD\,36705, SAO\,249286)
has been at the focus of much attention recently
(cf.\ Vilhu {\em et al}., 1993; Collier Cameron, 1995;
and references cited therein).
The ephemeris of Innis {\em et al}.\ (1988),
 Min = 2444296.575 + 0.51479$\times$E, is usually used (as here) for
reckoning phases.

The star was observed in $B$ and $V$ ranges on 12 nights
between Nov 3, 1994 and Jan 6, 1995
with the 20cm S-C telescope and DC photometer
of the Mt Molehill Observatory (cf.\ Bos, 1994).
Observations were also made using the automated photometer (`APT')
at the Kotipu Place Observatory on four nights between 26 Oct and 14 Dec, 1994,
using the $UBV$ filters provided with the SSP 5 `Optec'
photometer (cf.\ Hudson {\em et al.}, 1993). Most of this latter data come from
two nights 26 Oct and 1 Nov; data from 6 and 14 Dec
were prematurely terminated by an APT tracking fault.

Standard broadband differential photometric reduction procedures were followed
(cf.\ e.g.\ Budding, 1993).
The main comparison star was again HD 37297 ($V$ = 5.34, $B - V$ = 1.04,
$U - B$ = 0.85, sp.\ type K0III --- cf.\ SIMBAD). This comparison was regularly
checked against HD 37279 at Mt Molehill (Table 1), showing the stability of
HD 37297 to within 0.01 mag, except on Nov 9, when poorer weather affected
the data (Figure 1a, phase range 0.0 --- 0.4).
  At Kotipu Place HD\,37279 was checked
against HD\,35537, also a K0III star (Budding {\em et al.}, 1994).

%\begin{table}[h]
\begin{center}
\centerline{Table 1. Check star HD\,37279 (mean values)}
\begin{tabular}{cccccc}
\hline
Date & $n$ &  $V$ mag &  S.D. &  $B-V$ &   S.D. \\
\hline
     Dec 1992 - Jan 1993 &  14 &  7.429 & 0.007 &  0.258 & 0.006\\
     Nov 1993 - Jan 1994 &  47 &  7.432 & 0.009 &  0.257 & 0.009\\
     Nov 1994 - Jan 1995 &  82 &  7.432 & 0.008 &  0.257 & 0.009\\
\hline
\end{tabular}
\end{center}
{\footnotesize Here S.D. = standard deviation, Nov 09 1994 data not included}
%\end{table}
\vskip 3mm
The resulting data have been plotted up as follows: Figure 1 (a-c) the
Mt Molehill $V$ data, Figure 2 montage of the APT (preliminary) $U,B,V$ data.
Although none of these data sets are quite complete, they indicate
 trends in the variability towards the end of 1994
and beginning of 1995.
The main characteristics of the Mt Molehill
data sets are listed in Table 2.
Figure 2 results from
binning about 340 individual observations in each colour
into about 45 `normal' points.
\vskip 2mm
%\begin{table}[h]
\begin{center}
\centerline{Table 2. Variability of AB Dor --- amplitude and minimum phase}
\begin{tabular}{ccccc}
\hline
  & Max. & Min. & Amplitude & Phase \\
\hline
     Figure 1a &   6.81&    6.92&     0.11 &     0.50\\
     Figure 1b &   6.82&    6.92&     0.10 &     0.42\\
     Figure 1c &   6.81&    6.89&     0.08 &     0.41\\
\hline
\end{tabular}
\end{center}
\vfill \eject

\centerline{2}
\vskip 1mm
\epsfysize=11cm
\centerline{\epsffile{4203-f1.ps}}
\centerline{Figure 1. AB Dor: $V$ light curves from Mt Molehill}
\vskip 3mm

\epsfysize=11cm
\centerline{\epsffile{4203-f2.ps}}
\centerline{Figure 2. AB Dor: $U,B,V$ light curves from Kotipu Place APT}
\vfill \eject

\centerline{3}
\vskip 1mm

        The observed minima are associated with what is taken to
be spot B of Innis {\em et al}.\ (1988), rather than spot A,
which relates with the minima reported for the preceding year
     Anders (1994), Bos (1994) and Budding {\em et al}.\ (1994).
     What remains of spot A is perhaps the noisy
maximum near phase zero in Figures 1b,c, though
asymmetry is also noticeable in the shape of the main minimum.



     Figure 1b shows a definite change in the slope of the rise between
the observations made on November 19 and December 06 1994.  The minimum
of the light curve in Figure 1c is not as deep as that observed four
     weeks earlier (Figure 1b, Table 2).
 Because no clear minimum or maximum was observed in
     early November there is some uncertainty about the phase of minimum
     and the amplitude of the light curve then. It is, however, not
     less than 0.10 magnitudes and may have been as much as 0.12. The
phase of minimum in Figure 1a must be somewhere near 0.5, though it may be
closer to 0.4, as for Figures 1b,c.

AB Dor has remained at much the same brightness at maximum (6.82 $V$) as 12
     months earlier. Both Bos (1994) and Budding {\em et al}.\ (1994) observed 
a small
     drop in $B-V$ coinciding with the minima of November 1993. There
     appears to be some evidence of this in the 1994/1995 data as well,
but less clearly than before.

Incompleteness of the APT data-sets about the minimum
is frustrating, though it supports the slight
apparent deepening near phase 0.3 in December
1994, and a tendency of the curve to rise near phase
0.6 around that time (Figures 1b,c).
This could be interpreted as a tendency of the
minimum again to drift down in phase during the period
late October 1994 to the end of the year, as suggested also
for the preceding year's data (Budding {\em et al}., 1994).


\vskip 1cm
\beginauthors
\authorsrightcol
M. BOS, 
E. BUDDING,
G. HUDSON and 
R. HUDSON
Mt Molehill Observatory, Auckland; 
Carter and Kotipu Place 
Observatories, 
Wellington, New Zealand
\endauthors

\references

Anders, G.J., 1994, {\em IBVS}, No.\,3985

Bos, M., 1994, {\em IBVS}, No\,4111

Budding, E., 1993, {\em An Introduction to Astronomical Photometry},
 Cambridge Univ.\ Press, Cambridge \BIBCODE{1993iap..book.....B}

Budding, E., Hudson, G.\ and Hudson, R., 1994, {\em IBVS}, No\,4139

Collier Cameron, A., 1995, Preprint, submitted to {\em Mon.\
Not.\ Royal Astron.\ Soc.} \BIBCODE{1995MNRAS.275..534C}

Innis, J.\ L., Thompson, K., Coates, D.\ W.\ and Lloyd Evans, T.,
1988, {\it Mon.\ Not.\ Roy.\ Astron.\ Soc.}, {\bf 235}, 1411

Hudson, G., Hudson, R.\ and Budding, E., 1993,
{\em Proc.\ IAU Coll.\ 136}, (Posters) eds.\ I.\ S.\
Elliott and C.\ J.\
Butler, Dublin Institute for Advanced Studies, 107 \BIBCODE{1993ppsp.conf..107H}

Vilhu, O.\ Tsuru, T.\ Cameron, A.\ C., Budding, E., Banks, T.,
Slee, O.\ B., Ehrenfreund,
P.\ and Foing, B.\ H., 1993, {\em Astron.\ Astrophys.}, {\bf 278}, 467

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