\documentstyle[twoside, epsf]{article}

\input ibvs2.sty

\begin{document}

\IBVShead{5762}{2 April 2007}

\IBVStitletl{Rapid Changes in the Light Curve of the}{ Active, Late-Type Subgiant CF Octantis}

\IBVSauth{Innis, J.L.$^{1}$; Coates, D.W.$^{2}$; Kaye, T.G.$^{3}$}

\IBVSinst{Brightwater Observatory, 280 Brightwater Rd., Howden, TAS,
7054, Australia, email: brightwater@iraf.net} 

\IBVSinst{School of Physics, Building 27, Monash University, VIC, 3800, Australia}

\IBVSinst{Spectrashift, 404 Hillcrest, Prospect Heights, IL 60090, USA}

\SIMBADobjAlias{CF Oct}{HD 196818}

\GCVSobj{CF Oct}
\IBVStyp{ RS }
\IBVSkey{Photometry, spots}

\IBVSabs{B and V photometry of CF Octantis, obtained in 2006 July-2007 March,} 
\IBVSabs{showed the spot-wave underwent a rapid change near HJD 2454040.}
\IBVSabs{The amplitude changed from 0.12 mag to less than half this. Such a}
\IBVSabs{rapid change has not been reported previously for this active subgiant.}

\begintext

CF~Octantis (HD~196818) is a very active late-type (K0) subgiant showing strong Ca~II emission (e.g.\ Hearnshaw, 1979;
Innis et al., 1997) and a 20.15-d spot wave of varying amplitude (Innis et al., 1983; Lloyd Evans \& Koen, 1987;  
Pollard et al., 1989; Innis et al., 1997). The radial velocity data of Lloyd Evans (1986), Balona (1987), Collier
Cameron (1987a) and Innis et al.\ (1997) show no evidence for binarity. The star is active at radio wavelengths (Slee et
al., 1987a, 1987b; Vaughan \& Large, 1987), indeed it appeared as one of the stronger flaring microwave sources seen in
the Parkes survey. It also appears in the ROSAT bright source catalogue 
(Schwope et al., 2000).

Apart from the work mentioned above, CF~Oct has not been well studied, probably in part due to its high southern
declination. It was first noted as a variable star on the Bamberg Southern Sky Survey photographic plates (Strohmeier,
1967).  A recent reanalysis of the Bamberg material recovered the spot-wave light curve for the years 1964--1969, with
some data from 1970, 1971, and 1976, showing the overall light variation of the star from that time (Innis et al.,
2004).  This photographic material, and the photoelectric photometry noted above, showed that while the spot wave was
variable, the changes were slow, and often data from many rotations, or even at times from different seasons, could be
combined to produce reasonably well defined light curves. In contrast, our recent data, presented here, reveal the star
underwent a rapid change in the form of its spot wave in a very short interval, possibly also showing a low level of
continuous change.

We commenced observations of CF~Oct in mid 2006.  We used an ST7 CCD and motorised $BVR$ filter wheel on a 70-mm
diameter, 480-mm focal length refractor.  The field of view of the CCD was $0.8\times0.55$ degrees. (See Innis et al.,
2007, for more details of the equipment and method.) CF~Oct and the comparison star HD~196520 could be obtained on the
same frame.  HD~196520 was also used as a comparison star by Lloyd Evans et al.\ (1983), Collier Cameron (1987b),
Pollard et al.\ (1989) and Innis et al.\ (1997), and has not been seen to vary.  CF~Oct and HD~196520 are almost
identical in $B-V$, so that colour transformation corrections are negligible.  We use $B-V=1.07$ and $V=7.60$ (Innis et
al., 1997) for HD~196520.

CF~Oct was observed for a total of 38 nights between 2006 July and 2007 March. We collected four 45-second $B$ and four
30-second $V$ exposures in succession and averaged the measurements, so that each resultant data point represents an
equivalent 180- or 120-second integration in $B$ and $V$ respectively. We typically repeated this sequence at least
four times on a given night.  We have in total around 240~measurements (each composed of a 4-point average as noted) in
each of $B$ and $V$.  The resulting $V$-light phase plot, using the period of 20.15~d (from Pollard et al., 1989; Innis
et al., 1997) is shown in Figure~1.  On any given night the scatter in the data is not much greater than the nominal
$\pm0.01$~mag error bar shown in the top left of the Figure.  We have inspected the magnitude differences between the
comparison star HD~196520 and several fainter field stars, and find no evidence for long-term change greater than 0.01
or 0.02~mag. (We had originally intended using the star CPD~$-80$~966 as the check star, but our data have shown this to
be a red semiregular star, Innis et al., 2006.)  We conclude that the scatter seen in the phase plot was due to real
changes in CF~Oct.

\IBVSfig{8cm}{5762-f1.eps}{CF~Octantis $V$ light curve for 2006 July--2007 March, phased with the
known 20.15~d rotation period.  The symbol in the top left of the plot represent a typical error bar per point of
$\pm0.01$~mag. The scattered nature of the plot is due to a real variation in the star}
\IBVSfigKey{5762-f1.eps}{CF Oct}{phased V light curve}

The changes in CF~Oct are more easily seen in Figure~2, where we plot $V$ magnitude versus HJD{}. We also show two
least-squares fitted sine waves to better illustrate the changes. These are not intended to be fits to the data (the
star clearly does not have a pure sinusoidal variation) but are to assist in judging the phasing of the data when
inspecting the plot.  We fixed the periods of the sine waves to be 20.15~d, and allowed the amplitudes, mean levels and
phases to be determined in the fit.  We arbitrarily split the data at HJD~2454040 when fitting the two sine waves.

\IBVSfig{8cm}{5762-f2.eps}{CF~Octantis $V$ light curve versus HJD for 2006 July--2007 March (circles).
The lines represent two least-squares fitted sine curves, as a schematic representation of the data before and
after HJD 2454040.  It is clear that the light curve is variable, possibly continually variable, but that a significant
change occurred near the above noted date.  The symbol in the top left of the plot represent a typical error bar per
point of $\pm0.01$~mag}
\IBVSfigKey{5762-f2.eps}{CF Oct}{V light curve}

\IBVSfig{8cm}{5762-f3.eps}{Top panel: CF~Octantis $B-V$ light curve versus HJD{}. These are nightly
averaged points.  Lower panel: Nightly averaged $B-V$ versus $V$}
\IBVSfigKey{5762-f3.eps}{CF Oct}{B-V light curve}


\vspace{-22pt}
The amplitude of the first segment of data (pre HJD~2454040) is about 0.12~mag peak-to-peak, which is around twice
that of the later data.  It appears that both maximum and minimum light have changed over the course of the
observations, with maximum light being several hundredths of a magnitude fainter at the end of the data set compared to
the start.  The change in maximum light has the appearance of a step-like decrease near HJD 2454040.  Minimum light
appears to have brightened, but possibly in a more gradual manner, and may have been continually variable.

Such rapid changes in the light curve of CF~Oct have not been previously reported.  Possibly the starspots are currently
undergoing an interval of rapid change. It is also possible that the earlier published observations represented an
unusually stable interval of spot behaviour, although the photoelectric data cover the interval $\sim1979$ to
$\sim1989$.

Changes in the light curve of the fast-rotating, active star FK~Com have been interpreted as being due to either {\it
phase jumps}, when a new spot (or spot group) first appears around 90\deg\ in longitude away from an existing spot,
or as {\it flip-flops}\/ when a new spot first appears 180\deg\ away from a decaying spot (Ol\'ah et al., 2006).  
The recent behaviour of CF~Oct, with a contemporaneous variation in minimum and maximum light, may be suggestive of
similar types of changes.  Further analysis is planned.

Our nightly averaged $B-V$ data are shown in Figure~3.  The top panel shows $B-V$ versus HJD, while the lower panel
shows $B-V$ versus $V$.  A clear gradient is seen in the lower panel, which is similar to the spot-induced colour change
reported in Pollard et al.\ (1989) and Innis et al.\ (1997). These new 
data suggest CF~Oct may be slightly bluer at a
given $V$ magnitude compared to the 1980s-era photoelectric photometry, but small errors in the transformations may
equally well account for the differences.
 
We will continue to monitor this star. It would be of interest to obtain new spectroscopic observations of the Ca~II and
H$\alpha$ lines, and also see if the possible increased activity is manifested in the radio and X-ray spectral regions.

\vspace{2mm}

{\bf Acknowledgments: } We thank D. Partridge, S. Norris, and T. Moon for assistance with the construction of the
observatory.  We thank Doug George of Diffraction Limited for data-acquisition software support. This work has made use
of the SIMBAD database of the Stellar Data Centre (CDS) Strasbourg, the NASA ADS abstract database, and the
data-red\-uction packages IRAF (NOAA, USA) and MUNIWIN (by David Motl).

\vspace{-6pt}

\references

Balona, L.A., 1987, {\it SAAO Circ.}, {\bf 11}, 1

Collier Cameron, A., 1987a, {\it SAAO Circ.}, {\bf 11}, 13

Collier Cameron, A. 1987b, {\it SAAO Circ.}, {\bf 11}, 57

Hearnshaw, J.B., 1979, {\it Proc.\ IAU Colloq.}, {\bf  46}, 371

Innis, J.L., Coates, D.W., Dieters, S.W.B., Moon, T.T., Thompson, K., 1983, {\it IBVS}, No.\ 2386

Innis, J.L., Coates, D.W., Thompson, K., 1997, {\it MNRAS}, {\bf 289}, 515

Innis, J.L., Borisova, A.P., Coates, D.W., Tsvetkov, M.K., 2004, {\it MNRAS}, {\bf 355}, 591

Innis, J.L., Coates, D.W., Kaye, T.G., 2006, {\it PZP}, {\bf 6}, 29

Innis, J.L., Coates, D.W., Kaye, T.G., 2007, {\it PZ}, {\bf 27}, 1

Lloyd Evans, T., Koen, M.C.J., Hultzer, A.A., 1983, {\it SAAO Circ.}, {\bf 7}, 82

Lloyd Evans, T., 1986, {\it SAAO Circ.}, {\bf 10}, 11

Lloyd Evans, T., Koen, M.C.J,, 1987, {\it SAAO Circ.}, {\bf 11}, 21

Ol\'ah, K., Korhonen, H., K\H{o}v\'ari, Zs., Forg\'acs-Dajka, E., Strassmeier, K.G., 2006, {\it A\&A}, {\bf 452}, 303

Pollard, K.R., Hearnshaw, J.B., Gilmore, A.C., Kilmartin, P.M., 1989, {\it J. Astrophys.\ Astron.}, {\bf 10}, 139

Schwope, A.D., et al., 2000, {\it AN}, {\bf 321}, 1

Slee, O.B., et al., 1987a, {\it PASAu}, {\bf 7}, 55

Slee, O.B., et al., 1987b, {\it MNRAS}, {\bf 229}, 659

Strohmeier, W., 1967, {\it IBVS}, No.\ 178

Vaughan, A.E., Large, M.I., 1987, {\it PASAu}, {\bf 7}, 42

\endreferences

\end{document}
