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\IBVShead{4305}{11 March 1996}

\centerline{\ft OPTICAL OBSERVATIONS OF THE ACTIVE STAR FF CANCRI} 
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The star FF Cancri = GSC $1383\_600$ was discovered to be an eclipsing
binary by Pravec (1993), who found it to have a color of (V$-$R)=0.48 and
a brightness V=10.83-11.40. 

The automated 0.5-m.  telescope, Cousins R filter and CCD camera of the
Climenhaga Observatory of the University of Victoria (Robb and Honkanen,
1992) were used to make photometric observations of FF Cancri.  The
frames were bias subtracted and flat fielded in the usual manner using
IRAF \footnote{IRAF is distributed by National Optical Astronomy
Observatories, which is operated by the Association of Universities for
Research in Astronomy, Inc., under contract to the National Science
Foundation}.  The magnitudes were found from aperture photometry using
the PHOT package.  The x y pixel coordinates of each star for photometry
were found from inspection of a few frames and these positions were used
as starting points for the Gaussian centering option which precisely
centered the 7 arc second aperture on each star for each frame. 

From the Hubble Space Telescope Guide Star Catalog (Jenkner et al.,
1990) the coordinates and magnitudes of the comparison star are RA=08\fh
29\fmm 43\fs, Dec=17\degr 15\farcmm 38\farcss, V=10.6 and of the check
star are RA=8\fh 30\fmm 09\fs, Dec=17\degr 16\farcmm 19\farcss, V=12.5
(J2000).  The standard deviation of the difference between the check and
the comparison star during a night ranged from 0\fm 008 to 0\fm 024. 
The mean and standard deviation of the six nightly mean differential
R magnitudes are  $-$1\fm284$\pm$0\fm006 ensuring the constancy of both
comparison and check stars at this level.  The precision of the
differential variable star minus comparison star measurements are
expected to be at this level.  Due to the small field of view first
order differential extinction effects were negligible and no corrections
have been made for them.  No corrections have been made for the colour
difference between the stars to transform the R magnitude to a standard
system. 

Photometric observations were made between the 2nd and the 16th of
February 1996 UT.  By the method of Kwee and Van Woerden (1956)
Heliocentric Julian Dates of primary minima were found to be
2450124\fd 6402, 2450125\fd 9613 and 2450129\fd 9304 and secondary minima
occurred at 2450123\fd9750 and 2450126\fd6235.  The uncertainty in the
times of minima were formally about 0\fd0005 days, but this estimate
does not include an allowance for the asymmetry in the minimum, and thus
the true uncertainty is larger.  These minima unambiguously determine
the orbital period to be about 1\fd32.  Pravec's 1992 data include the
sharp shoulder at the end of the primary eclipse at a time we estimate
to be 2448985\fd 5006.  We have similar observations at 2450124\fd 7099
and 2450130\fd 0158; requiring an integral number of cycles to have
elapsed gives us the following ephemeris:


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\centerline{HJD of Primary Minima = 2450123\fd 3152 + 1\fd 32313 $\times$ E}
\hspace*{8.5cm}$\pm$\fd 0012\  \ $\pm$0\fd 00004
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Plots of the light curve for periods corresponding to one cycle less and
one cycle more than this period look significantly noisier. 


A plot of the 831 differential R magnitudes phased at this period is
shown in Figure 1 with different symbols for each of the different
nights.  The obvious out-of-eclipse modulation of the light curve
indicates that one or both of the stars are spotted, since proximity
effects would be symmetrical about the minima.  The points marked with a
+ represent data from a night 8 days earlier than the other data
and show a shift of a few hundredths of a magnitude in the depth of the
secondary minimum and at the beginning of the secondary minimum,
indicating that the cool spots are probably on the primary star. 

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Figure 1. Light curve of 1996 differential R data of FF Cancri
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The (V$-$R) color of Pravec (1993) and a cursory inspection of a spectrum
taken with the Dominion Astrophysical Observatory 1.8m telescope
indicate approximately a K0V spectral type for the primary star.  Using
``Binmaker2" (Bradstreet 1993) and a temperature of 4900 K for the
primary star of the system, we have constructed a model light curve which
fits our observed light curve and can be seen in Figure 2.  Using 
albedo and limb darkening coefficients appropriate for the assumed
spectral types the best fit we found was for an approximately K5V
secondary star in an orbit inclined at 83\degr.  Two large spots on the
primary star are necessary to fit the out-of-eclipse variations, and
this is consistent with a presumably synchronous rotation period and the
temperature of the stars.  A 3-dimensional diagram of the system is
shown in Figure 3.  Note that this is not a unique solution; in
particular the spot latitude is not well determined. 
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Figure 2. Light curve and a possible Binmaker2 model for FF Cancri
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Figure 3. Three dimensional diagram of a possible Binmaker2 model 
of FF Cancri at phase 0.43
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Further measurements will be very valuable to increase our knowledge of
this important system.  Photometric monitoring will be of interest since
the changes in the light curve shape are probably due to differential
rotation or active region evolution.  Radial velocity measurements will
be very important for the determination of the absolute dimensions of
the system, the distance, and the gamma velocity.  A careful spectral
classification would confirm the temperature of the primary star and
check for the presence of Ca H \& K emission.  We note that the star is
in the direction of Praesepe in the sky and current estimates of its
distance are too uncertain to rule out membership in that cluster, so
proper motion and the radial velocity measurements will test for the
star's membership in the cluster.  Although FF Cancri has not been
detected in X-rays or the extreme ultraviolet (EUV) (Pounds et al. 
1993, Malina et al., 1994), it should be expected to be a source
since it has large active regions.  The eclipse of these active regions
by the secondary star would help to measure the spatial extent of the
emitting region. 

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\beginauthors
R. M. ROBB
M. D. GLADDERS
Climenhaga Observatory 
Dept. of Physics and Astronomy
University of Victoria
Victoria, BC, Canada,  V8W 3P6
Internet: robb@uvastro.phys.uvic.ca
Internet: gladders@uvastro.phys.uvic.ca
\endauthors

\references

Bradstreet, D.H., 1993, Binary Maker 2.0 User Manual, Contact Software,
Norristown, PA 19401-5505, USA

Jenkner, H., Lasker, B., Sturch, C., McLean, B., Shara, M., Russell, J., 1990, 
{\it AJ}, {\bf 99}, 2082
  
Kwee, K.K. and Van Woerden, H., 1956, {\it Bull. Astr. Inst. Neth.}, {\bf 12}, 327

Malina, R.F., et al., 1994, {\it AJ}, {\bf 107}, 751

Pounds, K.A., et al., 1993, {\it MNRAS}, {\bf 260}, 77

Pravec, P., 1993, {\it IBVS}, No.\,3839
 
Robb, R.M.  and Honkanen, N.N., 1992, {\it A.S.P. Conf. Ser.}, {\bf 28}, 105,
Automated Telescopes for Photometry and Imaging, ed.  Adelman, Dukes and
Adelman
%Original: {\it A.S.P. Conf. Ser.}, {\bf 38},
%Corrected: Tue Sep 27 09:13:06 MEST 2005
   
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