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[vsnet-obs 2443] Re: CY UMa confirmed

> Since this object is no longer on any alert programme, we will not issue a
> CVC. Readers interested in the superhump behaviour of CY UMa should
> read a recent paper by D. Harvey and J. Patterson (Center for Backyard
> Astrophysics, Columbia University, USA), which appeared in PASP, 107, November 
> 1995. The authors report a superhump period P=0.07210 +- 0.00003 d.
> The orbital period of CY UMa (0.06798 +- 8x10-5 d) is discussed by I.G.
> Martinez-Pais and J. Casares in MNRAS, 275, 1995.

    Some readers might be interested in the following, which is extracted
from my article published in IBVS No. 4236.  The paper by Thorstensen et al.
referred here was already also published.  The orbital period determined
by Thorstensen et al. is subtly different from that by D. Harvey and
J. Patterson; I think the value by Thorstensen et al. might have been
better determined, owing to their longer baseline.
    Since there have been only very few observations of late superhumps
except those in VW Hyi, systematic observations of post-superoutburst
behavior would be fruitful.  If the present one really turns out to be a 
superoutburst, it may provide a good chance owing to its good seasonal
condition.

Regards,
Taichi Kato
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   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$3 pixels to one pixel was adopted.
An interference filter was used which had been designed to reproduce the
Johnson {\it V} band.  The exposure time was between 20 and 120 s depending
on the observing condition.  The frames were first corrected for standard
de-biasing and flat fielding, and were then processed by a
personal-computer-based aperture photometry package developed by the
author.  The differential magnitudes of the variable were determined
against a local standard star GSC 3446.344 (10$^{\rm h}$ 57$^{\rm m}$
05$^{\rm s}$.38 +49$^\circ$ 37$'$ 30$''$.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 light curve on Dec. 30 (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].

  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 by radial velocity study;
Mart\'inez-Pais, Casares (1995) gave an orbital period of 0.06795
$\pm$ 0.00008 day, and Thorstensen  et al. (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.


The author is grateful to P. Schmeer for notifying us 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.


References

Goranskij V. P. 1977, Astron. Tsirk. No. 942

Harvey S., Patterson J. 1995, PASP, submitted

Kato T., Fujino S., Iida M., Makiguchi N., Koshiro M. 1988, VSOLJ
  Variable Star Bulletin 5, 18

Mart\'inez-Pais I. G., Casares J. 1995, MNRAS 275, 699

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

Stellingwerf R. F. 1978, ApJ 224, 953

Thorstensen J. R., Patterson J. O., Shambrook A., Thomas G.
      1995, in preparation