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\IBVShead{4236}{29 August 1995}


\centerline{\ft OBSERVATIONS OF SUPERHUMPS IN CY UMa}
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\begintext


   CY UMa was discovered as a dwarf nova by Goranskij (1977).  Little
attention had been paid until regular visual monitoring by VSOLJ members
detected a long outburst in Jan. 1988 (Kato et al., 1988).  From the
analysis of visual and photographic light curve during this outburst,
they detected a possible superhump period of 0.0593 day, and concluded
that CY UMa belongs to SU UMa-type dwarf novae.  Since this detection of
superhumps was suspected to be severely affected by limits of accuracy
of visual and photographic observations, the author has been trying to
confirm superhumps of CY UMa by CCD photometry.  The observations
reported here were done during two long outbursts in Dec. 1991 -- Jan.
1992 and in Mar. 1993.


  The observations were carried out using a CCD camera (Thomson TH~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 noise and dead time, an on-chip summation of
3$\times$3 pixels to one pixel was adopted.
An interference filter was used which had been designed to reproduce the
Johnson {\it V} band.  The exposure time was between 20 and 120 s depending
on the observing condition.  The frames were first corrected for standard
de-biasing and flat fielding, and were then processed by a
personal-computer-based aperture photometry package developed by the
author.  The differential magnitudes of the variable were determined
against a local standard star GSC~3446.344 (10\fh 57\fmm
05\fss 38 +49\degr 37\farcmm 30\farcs 4 (J2000.0), {\it V}=12.9. The
position and magnitude were taken from the Guide Star Catalog).
The constancy of this comparison was checked against several stars in
the same field.

  The first outburst was covered from the terminal stage to its return
to near quiescence (Figure 1).  The  Dec. 30 light curve (Figure 2)
clearly shows superhumps with an amplitude of 0.18 mag.  A period analysis
of observations on Dec. 29--30 using the Phase Dispersion Minimization
(PDM) method (Stellingwerf 1978), after heliocentric correction and removing
a linear trend of decline, has yielded a superhump period of 0.0714 $\pm$
0.0005 day.  This observation clearly confirmed the SU UMa-type nature
of CY UMa.  Additional observations were performed on four nights from
Jan. 1 through Jan. 4 just after the star returned to near quiescent
brightness.  A period analysis gives a theta diagram (Figure 3), which
suggests persistence of the superhump period near P=0.0723 day and
possible periodicity near 0.0678 day.  Although irregular variation and
relatively low signal-to-noise ratio have made these periods less
confident, one may attribute the variability of the first period to
late superhumps and the second possible orbital humps [however, one should
note that the latter period may be affected by the one-day alias of
the first period].
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Figure 1. {\it V}-band light curve of CY UMa during a superoutburst
in Dec. 1991--Jan. 1992.  The zero point of the magnitude scale
corresponds to {\it V}=12.9.
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Figure 2. The light curve on Dec. 30, 1991.  Superimposed on  a slow decline, superhumps with an amplitude of 0.18 mag are clearly
seen.
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\centerline{3}
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Figure 3. Theta diagram (Stellingwerf 1978) of period analysis
for the post-outburst period Jan. 1--Jan. 4, 1991.  Two minima in the
theta diagram represent the periods of 0.0723 and 0.0678 day (see the
text for interpretation).
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Figure 4. The light curve on Mar. 5, 1993 (second outburst).
Doubly humped superhumps with a period of 0.0721 day are clearly seen.
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\centerline{4}
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  The second outburst was followed on one night, Mar. 5, 1993.
Doubly humped superhumps were clearly detected (Figure 4).
A period analysis with the same procedure as in the first outburst has
yielded a superhump period of 0.0721 $\pm$ 0.0003 day.
A small difference of superhump periods obtained during two different
superoutbursts may reflect intrinsic period variation
of superhumps.  From these observations we may safely conclude that the
superhump period of CY UMa is 0.0719 $\pm$ 0.0005 day, which was later
independently confirmed during the 1995 superoutburst by Harvey, Patterson
(1995), who gave a period of 0.0724 $\pm$ 0.0005 day.  The star was
recently investigated by two groups based on radial velocity study;
Mart\'{\i}nez-Pais, Casares (1995) gave an orbital period of 0.06795
$\pm$ 0.00008 day, and Thorstensen (1995) 0.06957 $\pm$ 0.00004 day.
The value of fractional superhump excess
($(P_{\rm SH}-P_{\rm orb})/P_{\rm orb}$) obtained by our superhump period
seems to support the latter period, but near coincidence of the former
period with one observed after the first outburst would require additional
confirmation of the true orbital period.

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The author is grateful to P. Schmeer for notifying him of the outbursts,
and J. R. Thorstensen and J. Patterson for providing their preprints.
Part of this work is supported by a Research Fellowship of the Japan
Society for the Promotion of Science for Young Scientists.

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\beginauthors
\authorsrightcol
Taichi KATO
Kyoto University
Sakyo-ku, Kyoto 606-01 Japan
\endauthors


\references

Goranskij, V. P., 1977, {\it Astron. Tsirk.},  No.\,942

Harvey, S., Patterson, J., 1995, {\it PASP}, submitted

Kato, T., Fujino, S., Iida, M., Makiguchi, N., Koshiro, M., 1988, {\it VSOLJ
  Variable Star Bulletin}, {\bf 5}, 18

Mart\'{\i}nez-Pais, I. G., Casares, J., 1995, {\it MNRAS}, {\bf 275}, 699

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 ApJ}, {\bf 224}, 953

Thorstensen, J. R., 1995, in preparation
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