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\IBVShead{4152}{27 January 1995}                      % Use it with dummy or empty arguments

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\centerline{\ft OBSERVATIONS OF SUPERHUMPS IN V1251 Cyg}
\centerline{\ft DURING THE 1991 SUPEROUTBURST}
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Variability  V1251 Cyg was discovered by Weber (1966), who reported an outburst
occurred in 1963.  Rather rapid decline resembles that of a fast nova, but the
nature and the identification in quiescent state has remained uncertain for a long
time.  Systematic visual monitoring of this variable star by amateur
astronomers started in the 1980's.  After a long series of negative
observations, the star was finally caught in its second historical
outburst by M. Moriyama on 1991 Oct. 25.54 UT at m$_{\it v}$=12.4 and by
P. Schmeer on 1991 Oct. 26.833 UT at m$_{\it v}$=12.7
(Moriyama and Schmeer 1991, Korth 1991).
The outburst was subsequently confirmed by CCD observation by the
author on Oct. 28.  The object was first observed in {\it BVI} bands.
Its conspicuous blue color made the dwarf nova classification most likely.


  Following this confirmation, the author obtained a time-resolved
{\it V}-band CCD photometry of this object on eight nights between Oct. 28
and Nov. 15.
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 20 and 120 s
depending upon the brightness of the variable.  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 (21\fh 40\fmm
58\fss 59 + 48\degr 41\farcmm 23\farcs 1 (J2000.0), {\it V}=10.1; the
position and the magnitude from the Guide Star Catalog),
whose constancy was confirmed using several check stars in the same
field.  The estimated error of single observation is 0.01 mag under
favorable condition.


  The overall light curve for the outburst is given in Figure 1, which
shows a smooth slow decline followed by a rapid return to quiescence.
From the time-resolved photometry, clear superhumps with a mean amplitude
of 0.24 mag were detected on Nov. 3 (see Figure 2);
this observation revealed the SU UMa-type nature of this object (Kato 1991).
Twelve superhump maxima were observed between Nov. 3 and 6, and by linear
regression of the superhump times, we could obtain a superhump period
of 0.0759 day.  A period analysis using the phase dispersion minimization
(PDM) method (Stellingwerf 1978) after removing the steady decline
yielded the best estimate of the superhump period of 0.07604 $\pm$ 0.00010
day.  An examination of a rather fragmentary light curve
on Oct. 28 could reveal only 0.05 mag variation (Figure 3), which
could not fit the above superhump period.  An examination of an image
on Nov. 2 taken at the expected superhump maximum again failed to show
a large amplitude ($\geq$ 0.1 mag) variation against the rest of the
images taken on the same night.  From these findings we concluded that
the full development of the superhumps in this object took 4 -- 7 days
from the outburst maximum.  This value is much longer than 2 -- 3 days
in typical SU UMa stars.
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\centerline{2}
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\centerline{\epsffile{4152-f1.ps}}
\centerline{Figure. 1.  General {\it V}-band light curve of V1251 Cyg. The}
\centerline{zero point of the relative magnitudes corresponds to {\it V}=10.1.}

\epsfysize=10cm
\centerline{\epsffile{4152-f2.ps}}
\centerline{Figure. 2. Light curve for nights from Nov. 3 through Nov. 6. The magnitude}
\centerline{is offset by 0.5 mag for each night. Superhumps are clearly seen.}
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\centerline{3}
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\centerline{\epsffile{4152-f3.ps}}
\centerline{Figure. 3. Light curve on 1991 Oct. 28. Only a small}
\centerline{amplitude ($\sim$ 0.05 mag) variation was present.  The large}
\centerline{scatter is due to the unfavorable sky condition.}
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  The star has been monitored since, and was recorded
again in superoutburst on 1994 Dec. 30 (Schmeer et al. 1995).
Apparent absence of normal outbursts between these two superoutbursts,
rarity of outbursts from a search in archival plates (Wenzel 1991),
the long interval ($\geq$ 3 years) between the two successive superoutburst,
a large outburst amplitude ($\sim$ 6.5 mag), and the slow development
of superhumps all make V1251 Cyg a close relative of WZ Sge stars
(Bailey 1979; Downes 1990) or TOADs (Tremendous Outburst Amplitude
Dwarf Novae; Howell 1993).  All the characteristics of V1251 Cyg
most resemble those of SW UMa, despite the fact that V1251 Cyg has
a much longer superhump period.

The author would like to point out one more peculiar feature in the
light curve.  Although one should be careful in comparing the visual
and CCD magnitudes, the available material shows that this dwarf nova
showed a rapid initial decline from m$_{\it v}$=12.4 (Oct. 25) to 13.3
(Oct. 28, this study).  The similar feature can be also found in the
light curve by Weber (1966).  This feature would be explained, in the
scheme of disk instability model, by the extra mass accumulated in the
accretion disk during quiescence in the absence of normal outbursts
(Osaki 1994), and would be a clue to understanding the peculiar
outburst pattern of WZ Sge stars.  A search for normal outbursts and
detailed observations of V1251 Cyg both during outburst
and quiescence are therefore highly encouraged.

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\centerline{4}
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The authors are grateful to M. Moriyama (VSOLJ) and P. Schmeer for
promptly notifying us of the outburst, and providing the visual
estimates.  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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% Call for macros \authors - \authorsrightcol - \endauthors
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Taichi KATO
Dept. of Astron., Faculty of Sci.
Kyoto University
Sakyo-ku, Kyoto 606-01 Japan

\endauthors

% Call for macro \references
\references

Bailey J., 1979, {\it MNRAS}, {\bf 189}, 41P

Downes R. A., 1990, {\it AJ}, {\bf 99}, 339

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

Kato T., 1991, {\it IAU Circ.}, No.\,5379

Korth, S., Moriyama, M., Schmeer, P., 1991, {\it IAU Circ.}, No.\,5380

Moriyama, M. and Schmeer, P., 1991, {\it IAU Circ.}, No.\,5377

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

Osaki Y., 1994, in ``Theory of Accretion Disks -- 2",
          ed W. J. Duschl, J. Frank, F. Meyer, E. Meyer-Hofmeister,
          W. M. Tscharnuter (Kluwer Academic Publishers, Dordrecht)

Schmeer P., Szentasko L., and Manning B., 1995, {\it IAU Circ.}, No.\,6123

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

Weber, R., 1966, {\it IBVS}, No.\,123

Wenzel W., 1991, {\it IBVS}, No.\,3689



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