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\IBVShead{4763}{3 September 1999}

\IBVStitletl{CCD Photometry of the 1999 February Superoutburst of }{ CY UM\lowercase{a}}

\IBVSauth{T.~Kato, K.~Matsumoto}

\IBVSinsto{Dept. of Astronomy, Kyoto University, Kyoto 606-8502, Japan, e-mail: tkato@kusastro.kyoto-u.ac.jp}

\SIMBADobj{CY UMa}
\IBVStyp{ dwarf nova }
\IBVSkey{Dwarf novae - photometry}

\begintext

CY UMa was discovered as a dwarf nova by Goranskij (1977). Following the initial claim of visual and photographic superhump detection by Kato et al.\ (1988), CCD observations independently revealed the presence of superhumps (Kato 1995; Harvey, Patterson 1995).

During the 1999 February superoutburst, starting on 1999 February 10 (E.~Muyllaert, $m_{\rm v}=12.5$, VSNET), we undertook time-resolved CCD photometry of CY UMa.

The observations were done using an unfiltered ST-7 camera attached
to the Meade 25-cm Schmidt-Cassegrain telescope. The exposure time
was 30 s. The images were dark-subtracted, flat-fielded, and
analyzed using the Java$^{\rm TM}$-based aperture and PSF photometry package
developed by one of the authors (TK). The flux of the variable was measured
relative to GSC 3446.96 (Tycho $V=10.84$, $B-V=0.38$; USNO $r$-magnitude
10.1), whose constancy was confirmed by comparison with GSC 3446.384
(USNO $r$-magnitude 12.4).

Figure 1 illustrates the overall light curve of the present observation.
Before February 22 (JD 2451232) individual observations are plotted;
after the decline, nightly averages with error bars are plotted instead.
The superoutburst plateau stage (February 13--20) was analyzed, after
subtracting the linear decline trend (0.14 mag\,d$^{-1}$), using the Phase
Dispersion Minimization (PDM) method (Stellingwerf 1978).

The result of period analysis is given in Figure 2. Although unavoidable
one-day aliases exist, we can safely choose, with the help of previous
period determinations, the correct period of $0.0722 \pm 0.0001$~d,
corresponding to the frequency of 13.85~d$^{-1}$.
The present best period agrees with previous reports within respective
errors: $0.0719 \pm 0.0005$~d (Kato 1995) and $0.07210 \pm 0.00003$~d
(Harvey, Patterson 1995).

Besides superhumps, we examined the post-superoutburst behavior, during
which some SU UMa-type dwarf novae are known to show rebrightenings.
No evidence of rebrightening was observed, both in our CCD monitoring
until 12 d past the steep decline, and visual monitoring reported to VSNET.

Figure 3 shows enlarged light curves of nightly runs. The typical error
of a single measurement was 0.01 mag on February 13 and 0.02 mag on other
nights. The amplitude of superhumps was the largest (0.30 mag) on February
13, and gradually decayed. On the declining branch from the superhump
maximum on February 13, quasi-periodic oscillations (QPOs) with a period of
10--15 m were visible. This modulation quickly decayed on subsequent nights.

\IBVSfig{10cm}{4763-f1.ps}{Overall light curve of CY UMa}

\IBVSfig{10cm}{4763-f2.ps}{Period analysis of CY UMa}

\IBVSfig{19cm}{4763-f3.ps}{Nightly light curve of CY UMa}

The authors are grateful to VSNET observers who reported timely visual
observations. This work is partly supported by the Grant-in-Aid for Scientific Research (10740095) of the Japanese Ministry of Education, Science, Culture, and Sports (TK). Part of this work is supported by a Research Fellowship of the Japan Society for the Promotion of Science for Young Scientists (KM). 

\references

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

Harvey D., Patterson J., 1995, {\it PASP}, {\bf 107}, 1055

Kato T., 1995, {\it IBVS}, No.\,4236

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

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

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