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\input ibvs2.sty

\begin{document}

\IBVShead{5157}{16 August 2001}

\IBVStitle{The SU UM\lowercase{a} Nature of V630 Cygni}

\IBVSauth{Nogami, D.$^{1,2}$; Buczynski, D.$^3$; Baba, H.$^{2,4}$; Kato, T.$^2$}

\IBVSinst{Hida Observatory, Kyoto University, Kamitakara, Gifu
506-1314, Japan,\\ \indent e-mail: nogami@kwasan.kyoto-u.ac.jp}

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

\IBVSinst{Conder Brow Observatory, Littlefell Lane, Lancaster LA20RQ
England, e-mail: denis@cb978iau.demon.co.uk}

\IBVSinst{Astronomical Data Analysis Center, National Astronomical
Observatory, Mitaka, Tokyo 181-8588, Japan,\\ \indent e-mail: hajime.baba@nao.ac.jp}

\SIMBADobj{V630 Cyg}
\IBVStyp{ UGSU }
\IBVSkey{dwarf nova, photometry}

\begintext

V630 Cyg was discovered and designated as S 4556 by Hoffmeister
(1949).  Romano (1966) observed this star and found that V630 Cyg shows
frequent short and long outbursts, which is very suggestive of an SU
UMa-type dwarf nova.  Then, the star has been regularly monitored as a
candidate SU UMa-type dwarf nova by a number of amateur observers.
Wenzel (1989) also detected apparent superoutbursts.  Bruch \& Schimpke
(1992) obtained an optical spectrum with the weak Balmer emission lines
which does not well agree with the normal behavior of SU UMa stars.
Cordova et al.\ (1981) reported that V630 Cyg was not detected during a
survey with {\it HEAO-1} in soft X-rays.  Nogami et al.\ (1997) measured
that the recurrence cycle of long outburst is 290 d and that of short
outburst is 30--50 d.

\begin{table}[hb]
\caption{The observation summary}
\begin{center}
\begin{tabular}{lccccccc}
\hline
\multicolumn1c{\lower6pt\hbox{Date}}   & HJD       & HJD     & Exposure & \lower6pt\hbox{Error$^2$} & Mean    & \lower6pt\hbox{$N^4$} \\[-4pt]
& start$^1$ & end$^1$ & time (s) &        & $V$ mag$^3$ & \\
\hline
17 August, 1996 & 50313.054 & 50313.060 &  90 & 0.01 & 2.419 & 5   \\
18 & 50313.958 & 50314.086 &  90 & 0.02 & 2.531 & 112 \\
19 & 50315.187 & 50315.293 &  90 & 0.02 & 2.719 & 93 \\
20 & 50316.209 & 50316.297 &  90 & 0.04 & 2.911 & 72 \\
21 & 50317.217 & 50317.222 &  60 & 0.03 & 4.035 & 5 \\
\phantom{0}5 September, 1996 & 50332.097 & 50332.103 &  90 & 0.25 & 5.949 & 5 \\
10 July, 1999 & 51370.497 & 51370.579 &  30 & 0.05 & 2.559 & 200 \\
11 & 51371.466 & 51371.549 &  30 & 0.05 & 2.647 & 200 \\
12 & 51372.470 & 51372.552 &  30 & 0.07 & 2.739 & 199 \\
\hline
\end{tabular}

\normalsize
\begin{tabular}{c@{ }l}
$^1$ & HJD${}-{}$2400000 \\
$^2$ & Nominal error for each point \\
$^3$ & Magnitude relative to the local standard star GSC 0318701786 (GSC mag = 12.18) \\
$^4$ & Number of frames \\
\end{tabular}
\end{center}
\end{table}

To investigate the nature of V630 Cyg, we carried out time-resolved
photometry during outbursts caught by P. Skalak (Vanmunster 1996) in
1996 and by Poyner (1999) in 1999.

In 1996, we performed the observations at Ouda Station, Kyoto
University.  A 60-cm reflector (focal length${} = 4.8$ m) and a Thomson TH
7882 CCD camera with a Johnson-$V$ filter attached to the Cassegrain
focus were used (for more information of the instruments, see Ohtani
et al.\ 1992).  In 1999, the observations were carried out at the
Conder Brow Observatory using an unfiltered CCD camera (SXL8) and a
33-cm Newtonian telescope.  Table~1 gives the journal of the
observations.

After standard de-biasing and flat fielding, the Ouda frames were
processed by a microcomputer-based aperture photometry package
developed by one of the authors (TK)\@.  The software used to reduce the
raw Conder Brow data was developed by Nick James in England and
performed standard de-biasing and flat fielding prior to processing
using an aperture-based photometry programme.  Magnitudes of V630 Cyg
were measured relative to the local comparison star GSC 0318701786
(GSC mag = 12.18).  The local check star GSC 0318700683 was used to
confirm the constancy of the comparison within 0\fmm02 during our
observations and measure the nominal error for each data point.

Figure 1 shows the light curve of the 1996 August outburst.  Since
Skalak noticed this outburst on August 10 (HJD 2450306), the outburst
lasted at least 11 days.  Periodic modulations were clearly superposed
on the slow decline trend (0.17 mag\,d$^{-1}$) between August 18 and
20.  After removing the decline trend, we performed a period analysis
by the phase dispersion minimization (PDM) method (Stellingwerf
1987).  The best estimated period is $0.0789\mathbin{(\pm}0.0004$) d (Figure
2a), and definite superhumps of this period is seen in Figure 2b,
confirming the SU UMa nature of V630 Cyg.

\IBVSfig{8.5cm}{5157-f1.ps}{Light curve of the 1996 August outburst}

V630 Cyg entered the rapid decline phase of the superoutburst and
became fainter by 1\fmm1 between our observations on August 20 and 21.
Two weeks after, on September 5, the relative magnitude of V630 Cyg was
$5.95 \pm 0.05$ mag, indicating that the amplitude of the superoutburst
is larger than 3\fmm53.  This is a normal value of an SU UMa-type
dwarf nova (see e.g.\ Nogami et al.\ 1997).

\IBVSfig{8.5cm}{5157-f2.ps}{(upper panel, a) Theta diagram of the PDM
analysis for the data between 1996 August 18 and 20, clearly
indicating $f=12.67 \pm 0.07$ d$^{-1}$ ($P=0.0789 \pm 0.0004$ d) as
the best estimated superhump period.  (lower panel, b) Superhump light
curve folded by the superhump period}

\IBVSfig{8.5cm}{5157-f3.ps}{(upper panel, a) Theta diagram of the PDM
analysis for the data obtained during the 1999 outburst.  The best
estimated period is $P=0.0783 \pm 0.0008$ d ($f=12.77 \pm 0.13$
d$^{-1}$), which is in accordance with the superhump period obtained in
the 1996 outburst within the error.  The other periods pointed by
peaks with higher significance are rejected by manual period
analysis.\\ (lower panel, b) Superhump light curve folded by
$P=0\fday0783$}

We again performed time-resolved photometry during a long outburst
detected by Poyner (1999) on 1999 July 8.  The theta diagram for the
de-trended data obtained between July 10 and 12 is exhibited in Figure
3a, suggesting the period $P=0.0783 \pm 0.0008$ d accordant with the
superhump period measured above.  Figure 3b shows the superhumps in
this superoutburst.

All outbursts reported to VSNET
(http://www.kusastro.kyoto-u.ac.jp/vsnet/) since 1995 are summarized in
Table 2.  Since V630 Cyg has been very closely monitored by many
amateur observers, it is a rare case that a superoutburst is missed.
The shortest recurrence cycle of superoutburst is 145 d, but seems to
vary to exceed several hundred days in these several years.  The
recurrence cycle of normal outburst is also unstable.  V630 Cyg may
become a key object for study of variation of mass transfer rate and
solar-type cycle in the secondary star.

\begin{table}

\begin{center}
\caption{Outbursts of V630 Cyg}
\begin{tabular}{cccl|cccl}
\hline
\lower6pt\hbox{JD start} & peak & \lower6pt\hbox{duration (d)} & \lower6pt\hbox{type} & \lower6pt\hbox{JD start} & peak & \lower6pt\hbox{duration (d)} & \lower6pt\hbox{type} \\[-4pt]
         & mag &              &      &      & mag &             &  \\
\hline
2449888 & 14.5 &  3 & normal   &  2450725 & 15.1 &  2 & normal \\
2449935 & 15.1 &  1 & normal   &  2450755 & 13.9 & 10 & super  \\
2449980 & 14.2 &  2 & normal   &  2450896 & 14.6 &  2 & normal \\
2449987 & 14.5 &  2 & normal   &  2451069 & 14.2 & 13 & super  \\
2450012 & 14.1 &  1 & normal   &  2451367 & 13.8 &  7 & super  \\
2450016 & 14.0 &  5 & normal   &  2451464 & 14.8 &  2 & normal \\
2450226 & 15.5 & 1$^a$ & normal & 2451688 & 14.5 & 1$^a$ & normal \\
2450274 & 14.2 & 4  & normal   &  2451823 & 14.5 & 2  & normal \\
2450306 & 14.0 & 12 & super    &  2451834 & 14.9 & 3 & normal  \\
2450610 & 14.3 &  8 & super    &  2451875 & 14.8 & 1$^a$ & normal \\
2450680 & 15.5 &  2 & normal   &  2451878 & 15.3 & 1$^a$ & normal \\
\hline
\end{tabular}

\normalsize
$^a$ single observation
\end{center}
\end{table}

\bigskip

The authors are very thankful to vigorous amateur observers for
reporting their useful observations to VSNET.

\references

Bruch, A., Schimpke, T., 1992, {\it A\&AS}, 93, 419

Hoffmeister, C., 1949, {\it Astron.\ Abh.\ Erg\"anzungshefte
z.\ d.\ Astron.\ Nach.}, {\bf 12}, No.\ 1, A3

Kinnunen, T., Skiff, B. A., 2000, {\it IBVS}, No.\ 4906

Nogami, D., Masuda, S., Kato, T., 1997, {\it PASP}, {\bf 109}, 1114

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

Poyner, G., 1999, vsnet-obs 21520 \\
(http://www.kusastro.kyoto-u.ac.jp/vsnet/Mail/obs21000/msg00520.html)

Romano, G., 1966, {\it Padova Publ.}, No.\ 132

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

Vanmunster, T., 1996, {\it Cataclysmic Variables Circular}, No.\ 103 (vsnet-obs
3397, see\\
http://www.kusastro.kyoto-u.ac.jp/vsnet/Mail/obs3000/msg00397.html)

Wenzel, W. 1989, {\it IBVS}, No.\ 3405

\end{document}
