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\IBVShead{4136}{28 December 1994}                      % Use it with dummy or empty arguments

% Call for macro \title {Title of the paper}
\centerline{\ft AK CANCRI -- A NEW LARGE-AMPLITUDE}
\centerline{\ft SU UMA-TYPE DWARF NOVA}
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AK Cnc was discovered by Morgenroth (1933), who classified it as a possible
short-period variable star.  After a long period of confusion, the object
was identified as a dwarf nova (for a review, see Wenzel 1993).
The peculiarity of this object was noted by Szkody and Howell (1992),
who identified strong Balmer and helium emission lines and a red continuum
in the spectrum of AK Cnc, and estimated the quiescent $M_{\rm V}$ value between
10.4 and 11.9.  The faintness of quiescent $M_{\rm V}$ and a large outburst
amplitude ($>6$ mag) make AK Cnc a good candidate for a TOAD
(Tremendous Outburst Amplitude Dwarf Nova; Howell 1993).
A search for a photometric period, however, has remained unsuccessful
(Howell et al. 1990).


On 1992 Jan. 13, the object was caught in one of its rare outbursts at
m$_V$=13.6 (Koshiro, private communication).  We obtained a V-band CCD
photometry of this star on three nights between Jan. 17 and Jan. 22.
The observations were carried out using a 60 cm reflector and a Thomson
TH7882 chip (576$\times$384 pixels) at Ouda Station, Department of
Astronomy, Kyoto University (for a description of the instruments
see Ohtani et al. 1992).  The exposure time was between 30 and 60 s
to avoid saturation from the strong moonlight.  The frames were first
corrected for standard de-biasing and flat fielding, and were then
processed by a microcomputer-based automatic-aperture photometry package
developed by the author.  The differential magnitudes of the variables
were determined using a local standard star (C$_1$: 08\fh 55\fmm
15\fss 42 +11\degr 14\farcmm 53\farcs 8 (J2000.0), {\it V}=13.3), whose
constancy was confirmed using a check star (C$_2$: 08\fh 55\fmm
26\fss 05 +11\degr 20\farcmm 04\farcs 5 (J2000.0), {\it V}=15.1).
Total number of useful frames was 1007.


The resulting overall light curve is shown in Figure 1.  The zero-point
corresponds to {\it V}=13.3.  A general trend
of slow linear decline is evident.  Superimposed on this decline,
superhumps with an amplitude of 0.18 mag were detected on all
nights.  The star was thus for the first time identified as being an
SU UMa-type dwarf nova.

A representative light curve is shown in Figure 2; a large
scatter in the light curve is caused by a high sky background due to
the proximity of the object to the nearly full moon.  A period analysis
using the phase dispersion minimization (PDM) method (Stellingwerf 1978)
implemented in the IRAF package after removing the steady decline
yielded the best estimate of the superhump period of
0.06735 $\pm$ 0.00005 day.
A light curve folded on this period is shown in Figure 3.  Each point
represents an average of 0.05 phase bin and its standard error.
This clearly demonstrates all the characteristics of fully grown superhumps:
a rather steep rise to maximum, slower decline, and a 
broader secondary maximum around superhump phase 0.4 -- 0.5.  The superhump
makes AK Cnc a member of short orbital-period SU UMa-type dwarf novae.
This picture is in good agreement with the spectroscopic features
described by Szkody and Howell (1992).

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\centerline{2}
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\centerline{Figure 1. General {\it V}-band light curve of AK Cnc.}
\centerline{The zero point of the relative magnitudes corresponds to {\it V}=13.3.}

\epsfysize=10.6cm
\centerline{\epsffile{4136-f2.ps}}
\centerline{Figure 2. Sample light curve (1992 Jan. 18).}
\centerline{Superhumps with an amplitude of 0.18 mag are clearly seen.}
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\centerline{3}
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\centerline{\epsffile{4136-f3.ps}}
\centerline{Figure 3. Light curve folded on the superhump period of 0.06735 day.}
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The outburst characteristics of this dwarf nova is still uncertain.
Wenzel (1993) gives five outburst records from Sonneberg plates.
The second one in his table (1993 March 8 -- 15) is clearly a
superoutburst.  Further monitoring for outbursts of this dwarf nova
is encouraged in order to determine its true outburst
frequency and number ratio of super- and normal outbursts,
both of which would also be good indicators for discriminating TOADs
from other dwarf novae.

The author is grateful to Masami Koshiro (VSOLJ) for notifying us
of the outburst.
Part of this work was supported by a Research Fellowship of the Japan
Society for the Promotion of Science for Young Scientists.

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% Call for macros \authors - \authorsrightcol - \endauthors
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Taichi KATO
Dept. of Astronomy, Faculty of Sci.
Kyoto University
Sakyo-ku, Kyoto 606-01 Japan
\endauthors

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\references

Howell, S. B. 1993, in {\it Cataclysmic Variables and Related Physics},
  ed O. Regev, G. Shaviv, Annals of the Israel Physical Society,
  Vol. 10, p.67

Howell, S. B., Szkody, P., Kreidl, T. J., Mason, K. O., and
  Puchnarewicz, E. M. 1990, {\it Publ. Astron. Soc. Pacific},
  {\bf 102}, 758

Morgenroth, O. 1933, {\it Astron. Nachricht.}, {\bf 249}, 383

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

Stellingwerf, R. F., 1978, {\it Astrophys. J.}, {\bf 224}, 953

Szkody, P. and Howell, S. B. 1992, {\it Astrophys. J. Suppl.},
       {\bf 537}, 547

Wenzel, W. 1993, {\it IBVS}, No.\,3921




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