[vsnet 754] preprint on CC Cnc

[Taichi Kato <tkato@kusastro.kyoto-u.ac.jp>]

   We have submitted the following article to PASJ.  The filgures in
PostScript are available at the VSNET anonymous ftp.

   ftp://ftp.kusastro.kyoto-u.ac.jp/pub/vsnet/preprints/CC_Cnc

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\centerline{New SU UMa-Type Dwarf Nova CC Cancri}
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\centerline{Taichi KATO and Daisaku NOGAMI}
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\centerline{\it Department of Astronomy,
Faculty of Science, Kyoto University,
Sakyo-ku, Kyoto 606-01}
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\centerline{\it E-mail (TK): tkato@kusastro.kyoto-u.ac.jp}

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% \centerline{(Received ; Accepted )}
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\centerline{\bf Abstract}

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{\bf Key words:} Stars: cataclysmic ---
Stars: dwarf novae ---
Stars: individual (CC Cnc)

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\noindent{\bf 1.~ Introduction}
\bigskip

  CC Cnc was discovered as a dwarf nova by Jankovics (1972), who discovered
four outbursts from Byviskan Observatory plates.  CCD time-resolved photometry
by Howell et al. (1990) during quiescence was suggestive of an orbital
modulation near their length of the data ($\sim$ 1.4 h) but longer time
coverage is needed to confirm any possible periodicity.  Spectroscopic
study by Munari et al. (1990) yielded a period of 0.0942 day,
entitling CC Cnc a rare dwarf nova in the period gap, in which the observed
number density of cataclysmic variables (CVs) is markedly decreased
(e.g. Whyte and Eggelton 1980).  The period gap is believed to arise from
the cessation of mass-transfer as a result of shrinkage of the secondary
star inside the Roche lobe, by loosing the magnetic braking mechanism when
the secondary star becomes fully convective during the course of the standard
CV secular evolution (Robinson et al. 1981).  Confirmation of the claimed
orbital period is highly desired since this object may provide a key in
understanding the precise mechanism of cessation of mass-transfer,
or suggested evolutionary paths of creating CVs inside the period gap
(cf. Rapapport et al. 1984).
We observed CC Cnc during three bright outbursts in order to
detect possible superhumps, which are characterstic of SU UMa-type dwarf
novae (Warner 1985; Warner 1995), thereby leading to unique clarification
of the nature of the object.

\bigskip
\noindent{\bf 2.~ Observation of CC Cnc during the 1996 May Outburst}

\bigskip
Observations were carried out using a CCD camera (Thomson 7882,
576 $\times$ 384 pixels)
attached to the Cassegrain focus of the 60-cm reflector (focal length~=~4.8~m)
at Ouda Station, Kyoto University (Ohtani et al. 1992).
To reduce the readout dead time, an on-chip summation of 2$\times$2
to one pixel was adopted except for the 1993 observations, for which a
summation of 3$\times$3 were adopted.  An interference filter was used which
had been designed to reproduce the Johnson {\it V} band.

\pvh
The exposure time was between 90 and 240 s depending on the transparency of
the sky; the dead time between exposures was typically 12 s.
A total of 375 useful object frames were obtained during three outbursts
of this object.  The journal of the observations is summarized in table 1.

\pvh
These frames were, after corrections for standard de-biasing and flat fielding,
processed by a microcomputer-based aperture and PSF photometry package
developed by on of the authors (T.K).  The differential magnitudes of the
variables were determined using the local standard star (C$_1$ in figure 1).
The magnitude of the comparison star was
determined to be $V=11.90$ by using a secondary standard star field
containing CC Cnc (Misselt 1996).
A comparison of the local standard star with a check star
(C$_2$ in figure 1) in the same field has confirmed the constancy of the
standard during a run, and gives the expected standard error in the
differential magnitudes for the variable as 0.03 mag for a single
frame on fine nights.  A relatively large error is caused by the
high sky background owing to the moon and citylight.

\bigskip
\noindent{\bf 3.~ Detection of Superhumps in CC Cnc}

\bigskip
We first describe the best observed 1996 May outburst.  The overall light
curve of the 1996 May outburst constructed from our CCD observations is shown
in figure 2.  Since CC Cnc was reported to be first caught in outburst
on May 10 (Mattei 1996), we may conclude that the present outburst lasted
at least 8 days.  A nightly decline rate of 0.13 mag d$^{-1}$ is
characteristic to those of superoutbursts of SU UMa-type dwarf novae.
A representative light curve on May 15 (figure 3) clearly
shows superhumps with an amplitude of 0.23 mag.  CC Cnc is thus confirmed
to be a new member of SU UMa-type dwarf novae.
After heliocentric correction and removal of a linear trend of decline,
a period analysis was applied to observations for the period of
May 14 -- 18 using the Phase Dispersion Minimization (PDM) method
(Stellingwerf 1978).  The resultant theta-diagram is shown in figure 4,
which shows a number of minima resulting from one-day aliasing due to the
short nightly coverage.  The lowest minimum is seen at a frequency of 11.98
$\pm$ 0.02 d$^{-1}$ which corresponds to a period of 0.0835 $\pm$ 0.0002 d.
Two adjacent minima correspond to 0.0771 $\pm$ 0.0002 d and 0.0911 $\pm$
0.0003 d.

\pvh
Similar superhumps were also detected during the 1993 and 1995 outbursts,
though with poorer statistics (figure 5 for the 1993 outburst).  PDM
analysis of two-night dataset of the 1993 outburst yielded periods of
0.076 d and 0.082 d (figure 6).  From these results of period analyses,
we have adopted the most likely superhump period of 0.0771 $\pm$ 0.0002 d,
though its one-day aliases of 0.0835 $\pm$ 0.0002 d is also acceptable.

\bigskip
\noindent{\bf 4.~ Discussion}

\bigskip
  The variable is thus confirmed to be a new member of SU UMa-type
dwarf novae.  The resultant superhump period of CC Cnc (0.0771 or 0.0835
d = 1.85 or 2.00 hr) seems to preclude the possibility of a dwarf nova
amid the period gap suggested by Munari et al. (1990).  Although secure
identification of the alias should await future observations, CC Cnc now
seems to be a rather normal SU UMa-type dwarf nova judging from its orbital
period. Outburst properties of CC Cnc has not been well determined due to
its faintness.  Since the 1995 March and 1996 May superoutbursts were
separated by $\sim$ 400 d, we may suggest a supercycle of CC Cnc to be
$\sim$ 400 d or its N-th.  This value also seems to be typical for an
usual SU UMa-type dwarf nova (Warner 1995).  Searches for dwarf novae
amid the period gap should require further moments of discovery.

\bigskip
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The authors are grateful to J. A. Mattei of the AAVSO and J. Pietz for
promptly notifying us of the outburst.

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{\bf References}

Downes D. A., Shara M. M. 1993, PASP 105, 127

Howell S. B., Szkody P., Kreidl T. J., Mason K. O., Puchnarewicz E. M.
     1990, PASP 102, 758

Jancovics I. 1972, IBVS No. 840

Mattei J. A. 1996, AAVSO NEWS FLASH No. 18

Misselt K. A. 1996, PASP 108, 146

Munari U., Bianchini A., Claudi R. 1990, IAUC No. 5024

Ohtani H., Uesugi A., Tomita Y., Yoshida M., Kosugi G., Noumaru J.,
     Araya S., Ohta K. et al. 1992, Memoirs of the Faculty
     of Science, Kyoto University,
     Series A of Physics, Astrophysics, Geophysics and Chemistry
     38, 167

Rapapport S., Verbunt F., Joss P. C. 1984, ApJ 275, 713

Robinson E. L., Barker E. S., Cochran A. I., Cochran W. D>, Nather R. E.
     1981, ApJ 251, 611

Stellingwerf R. F. 1978, ApJ 224, 953

Warner B. 1985, Interacting Binaries, ed P. P. Eggelton,
          J. E. Pringle (D. Reidel Publishing Company, Dordrecht)
          p367

Warner B. 1995, ApSS, 226, 187

Whyte C. A., Eggelton P. 1980, MNRAS 190, 801

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{\bf Fig. 1.~} Finding chart of CC Cnc drawn from a CCD image.
The north is up, and the field of view is about 10$^{'}\times$7$^{'}$.
The comparison star (C$_1$), check star (C$_2$) and CC Cnc (V) are
marked.  The J2000.0 coordinates of CC Cnc are
08$^{\rm h}$ 36$^{\rm m}$ 19$^{\rm s}$.32
+21$^\circ$ 21$'$ 05$''$.2 (Downes, Shara 1993).

{\bf Fig. 2.~}  Overall {\it V}-band light curve of CC Cnc during
the 1996 May superoutburst.  A slow linear decline followed by a rapid
one is characteristic of a superoutburst of SU UMa-type dwarf novae.

{\bf Fig. 3.~}  Enlarged light curve on 1996 May 15.  A superhump
with an amplitude of 0.23 mag can be clearly seen around HJD 2450218.96.
A subtle rising trend at the end of the observation suggests a period
of $\sim$ 0.08 d.

{\bf Fig. 4.~}  Theta diagram (Stellingwerf 1978) of period analysis
for observations between 1996 May 14 and 18.  The lowest minimum is seen at
a frequency of 11.98 $\pm$ 0.02 d$^{-1}$ which corresponds to a period of
0.0835 $\pm$ 0.0002 d.  Two adjacent minima correspond to 0.0771 $\pm$
0.0002 d and 0.0911 $\pm$ 0.0003 d.

{\bf Fig. 5.~}  Overall {\it V}-band light curve of CC Cnc during
the 1993 February superoutburst.  A scatter in the light curve is primarily
caused by superhumps.

{\bf Fig. 6.~}  Theta diagram of period analysis for the 1993 February
superoutburst.  A period of 0.076 d (frequency 13.1 d$^{-1}$) represents
a likely superhump period.

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