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\IBVShead{5444}{17 July 2003}

\IBVStitle{Period Change in an RS CV\lowercase{n} BINARY - what drives CF T\lowercase{uc}?}

\IBVSauth{INNIS, J. L.$^1$; COATES, D. W.$^2$; THOMPSON, K.$^2$; THOMSON, R.$^2$}

\IBVSinst{43 Ash Drive, Kingston, Tasmania, 7050, Australia; e.mail: j.innis@aip.org.au}
\IBVSinst{School of Physics and Materials Engineering, Building 27, Monash University, Melbourne, Victoria 3800, Australia; e.mail: denis.coates@spme.monash.edu.au, skyburnett@hotkey.net.au}

\SIMBADobjAlias{CF Tuc}{HD 5303}
\IBVStyp{ RSCVn }
\IBVSkey{Period change}

\IBVSabs{The active binary CF Tuc has shown clear period changes over the last}
\IBVSabs{two decades. The current period has been stable since about 1995.}


\begintext


CF Tuc is an RS CVn eclipsing binary of period $ \sim $2.8 d.  Its orbital 
period changes, as was first noted by Thompson, Coates and Anders (1991) and 
further discussed by Anders et al (1999) in terms of the mechanism proposed by 
Applegate (1992) for orbital period modulation of active close binaries.  
Anders et al. (1999) predicted that this mechanism would probably lead to a 
period shift for CF Tuc in the subsequent two years, and an increase in the 
spot-wave amplitude for the cooler stellar component.

We have timed two primary eclipses for CF Tuc in early 2000, via CCD 
photometry at Monash University, and have also obtained high-resolution 
spectra taken nine hours apart in 2002 June, as service observations using the 
University College London Echelle Spectrograph (UCLES) at the coude focus of 
the Anglo Australian Telescope (AAT).  Initial measurements of the radial 
velocities of the stellar components using these spectra gives another epoch 
of primary eclipse.

The observations of 2000 yield an epoch of HJD 2451605.099$\pm$0.003, and 
those of 2002 June HJD 2452452.742$\pm$0.020.  The second value was obtained 
independently by two of us, using cross-correlations between the spectrum of 
CF Tuc and that of a radial-velocity standard star ($\beta$ Hyi).  The 
Heliocentric radial velocities of the hotter star were determined to a 
precision of 2.0 km/s (sample standard deviation), which leads to a 
precision of 0.007 in the orbital phase,  and which corresponds in time to 
0.02 d.  The radial velocity was converted to orbital phase using fits to the 
radial-velocity data of Collier Cameron (1987) and Balona (1987).

The figure shows the $O-C$ graph for CF Tuc using the ephemeris given in Anders 
et al. (1999): 2444219.270 + 2.797715 $\times$ E.  The latest two data are obtained 
as above, the remainder are from Anders et al. (1999).  The predicted shift in 
period has clearly not happened.  The period remained essentially constant at 
2.797492~d from about 1995 to mid 2002.  Based on this information the current 
ephemeris would be: 2450351.825 + 2.797492 $\times$ E.

Clearly, more eclipse timings and multicolour photometry are needed for this 
interesting binary, so that the mechanisms involved in its period changes can 
be better understood.  We encourage southern observers to include this system 
on their observing programmes.

We thank the AAO for the UCLES service observations used to obtain the radial 
velocity measurements, which were carried out by S. Ryder.  The spectral 
analysis was performed using the IRAF package from the US National Optical 
Astronomy Observatories.

\newpage

\IBVSfig{10cm}{5444-f1.eps}{The O-C diagram for CF Tuc from 1978 to 2002}
%%\IBVSfigKey{5444-f1.eps}{CF Tuc}{O-C diagram}

\vskip 1cm

\references

Anders, G. J., Coates, D. W., Thompson, K., Innis, J. L., 1999, {\it MNRAS}, {\bf 310}, 377

Applegate, J. H., 1992, {\it ApJ}, {\bf 385}, 621

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

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

Thompson, K., Coates, D. W., Anders, G., 1991, {\it Proc Astron Soc Aust}, {\bf 
9}, 283

\endreferences

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