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\IBVShead{4155}{2 February 1995}                      % Use it with dummy or empty arguments

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\centerline{\ft DISCOVERY OF THE SECOND PG~0943+521-TYPE}
\centerline{\ft DWARF NOVA V1159 Ori}
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  This star was discovered to be variable by Wolf and Wolf (1906), called
36.1906, and then suggested by Kippenhahn (1953) to be a possible member
of U Gem stars.  In the New Catalogue of Suspected Variable Stars
(Kukarkin et al. 1982), it was designated as NSV~02011 with a brightness
range of m$_{pg}$ = 13.3 -- 14.2.  The subsequent photographic observation
by Natsvlishvili (1984) revealed that its magnitude varied from 12.5
at maximum to 16.6 at maximum, and it was finally named V1159 Ori of
INS: type in the 68$^{th}$ namelist of variable stars (Kholopov et al. 
1987). 


Jablonsky and Cieslinski (1992) made spectroscopic and photometric
observations and proposed its orbital period of 0.05890 $\pm$ 0.00001
days, which suggests that V1159 Ori is a good candidate for an SU UMa star.
Their observations, however, could not reveal superoutbursts or
superhumps expected for this class of CVs. The peculiar nature of
V1159 Ori was already suggested by their estimate of short outburst cycle
length of 4.35 days together with its small outburst amplitude.
VSOLJ (Variable Star Observers League in Japan) started CCD and
visual observations, part of which was published by Kiyota (1993).
These observations indicated presence of bright (reaching m$_{V}
\sim$12.1) outbursts, some of which bore the characteristics of
superoutbursts.

The present outburst of V1159 Ori was first detected by the RoboScope,
Indiana University Automated Photometric Telescope (Honeycutt and Turner
1992). It was independently discovered on Dec. 23 at m$_{V}$=12.9 by
M. Yamada (VSOLJ; private communication).  We made photometric
observations on 1994 Dec. 23 and 24 using CCD camera (Thomson,
TH7882CDA, 576 $\times$ 384 pixels with 23 $\mu$m square pixel size)
attached to the Cassegrain focus of 0.6-m reflector with Johnson V-band
filter at the Ouda Station, Kyoto University (Ohtani et al. 1992).
The mode of 2 $\times$ 2 on-chip summation was employed.
A total of 225 frames were taken between 23.154 and 23.270 UT
under clear sky, and 57 frames between 24.144 and 24.264 UT
interrupted for 82 minutes by clouds.  The exposure time
was 30 seconds on Dec. 23 and varied between 60 and 180 seconds on Dec. 24.

We reduced the data using the personal-computer-based aperture photometry
package developed by one of the authors (T.K.).
This package automatically subtracts bias-frames, applies flat fielding
and enables us to estimate the instrumental magnitudes.
The aperture size was 9" in radius. The sky level was determined from
pixels whose distance from the individual objects are between 24" to 48".
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\centerline{2}
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\centerline{Figure. 1. {\it V}-band light curve of V1159 Ori on 1994 Dec. 23.}
\centerline{The zero point of the relative magnitudes corresponds to {\it V}=11.99.}

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\centerline{\epsffile{4155-f2.ps}}
\centerline{Figure. 2. The theta diagram by PDM on the}
\centerline{data of V1159 Ori on 1994 Dec. 23 and 24.}
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\centerline{3}
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Figure 1 shows the light curve of differential magnitude between
V1159 Ori and a comparison star on Dec. 23.  The comparison star is
``c'' in Jablonsky and Cieslinski (1992), whose V magnitude is given
as 11.99 in thier Table 1.  A decay of amplitude of humps is seen
in the figure and the similar modulation was also observed in
PG~0943+521 (Nogami et al. 1995).  We analysed the light curve,
using phase dispersion minimization (PDM) method (Stellingwerf 1978)
implemented in IRAF package (IRAF is distributed by National Optical
Astronomy Observatories, U. S. A.). Figure 2 shows the $\Theta$
diagram, whose abscissa is frequency (day$^{-1}$). The lowest
minimum point in $\Theta$ corresponds to 0.06764 days. Robertson
et al. (1995) independently detected the same hump features
and reported that they are repeated with a period of 0.067 $\pm$ 0.001 days.

The difference in the photometric and spectroscopic periods confirmed
the superhump nature of these humps.  The outburst observed by Jablonsky
and Cieslinski (1992) did not show superhump-like modulation and corresponds
to normal outburst of usual SU UMa-stars.  Thus, V1159 Ori was unambiguously
identified to be an SU UMa-type dwarf nova with a superhump period of
0.06764 $\pm$ 0.0001 days.  The present discovery of superhumps in
V1159 Ori, together with its extreme shortness of its outburst recurrence
time, its long duty cycle (Jablonsky and Cieslinski 1992, Kiyota 1993),
and its extremely short supercycle (recurrence time of superoutbursts)
(Robertson et al. 1995), has established a new subgroup of peculiar
dwarf novae whose prototype is PG~0943+521 (Kato and Kunjaya 1995,
Misselt and Shafter 1995, Osaki 1995).

The fact that the supercycle of V1159 Ori (44.5 days, Robertson et al.
1995) is almost equal to that of PG~0943+521 (43 days) is
possibly not accidental.  According to Osaki (1995), the shortest supercycle
predicted by disk-instability theory under a given strength of the tidal
torques is about 40 days and the supercycle
is insensitive to the mass transfer rate from the secondary around
this minimum value.  However, as first pointed out in the case of
PG~0943+521 by Kato and Kunjaya (1995) and later confirmed
by numerical simulation by Osaki (1995), a dwarf nova with minimum
supercycle length should have much ($\sim$ ten times) larger mass
transfer rate than those of usual SU UMa-type dwarf novae, near
the borderline between nova-like stars and SU UMa-type dwarf novae.
For a dwarf nova with an orbital period below the period gap, it is
generally believed that its mass transfer is powered by the
gravitational-wave radiation, that is, its rate is mainly dependent
on the orbital period. The shortness of the orbital period of V1159 Ori
(0.05890 days, Jablonsky and Cieslinski 1992) and the superhump period
of PG~0943+521 (0.06549 days, Kato and Kunjaya 1995) implies small
mass transfer rates in the present scheme of CVs, which are clearly in
conflict with such large mass-transfer rate expected from their
outburst behavior.


The present discovery of ``another" PG~0943+521 star suggests that
this type of CVs may not considered to be unique but form a larger
population in CVs than ever expected.  In this case, the present picture
of the mechanism of mass transfer and the evolution of CVs below
the period gap would unavoidably be modified.
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\centerline{4}
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The authors are grateful to the VSOLJ (Variable Stars Observers League
in Japan) members for supplying us of visual and CCD estimates, and
especially M. Yamada for notifying us of the outburst. Thanks also
to Y. Osaki, K. A. Misselt and R. K. Honeycutt for sending us their
preprints. We acknowledge R. K. Honeycutt for opening the real-time
light curve by RoboScope to public through the World-Wide Web.
This research has been partly supported by Research Fellowship of the
Japan Society for the Promotion of Science for Young Scientists (T.K.).
\vskip 1cm


% Call for macros \authors - \authorsrightcol - \endauthors
\beginauthors
\authorsrightcol
Daisaku NOGAMI
Taichi KATO
Seiji MASUDA
Ryuko HIRATA
Dept. Astron., Faculty of Sci.
Kyoto University
Oiwake-cho, Kitashirakawa
Sakyo-ku, Kyto 606-01 Japan
\endauthors

% Call for macro \references
\references

Honeycutt, R. K., and Turner, G. W., 1992, in {\it Robotic Telescopes in the
1990's}, ed. A. Filippenko, (San Francisco, ASP), 277

Jablonsky, F. J., and Cieslinski, D., 1992, {\it Astron. Astrophys.},
{\bf 259}, 198

Kato, T., and Kunjaya, K., 1995, {\it Publ. Astron. Soc. Japan}, in press

Kholopov, P. N., Samus, N. N., Kazarovets, E. V., and Kireeva, N. N.,
1987, {\it Inf. Bull. Var. Stars}, No.\,3058

Kippenhahn, R., 1953, {\it Astron. Nachr.}, {\bf 281}, 153, (H4)

Kiyota, S., 1993, in {\it ``Variable Stars"} (in Japanese) Japan Variable Star
Study Association, Vol. 23

Kukarkin, B. V. et al., 1982, in {\it \lq\lq New Catalogue of Suspected
Variable Stars''}, ed. P. N. Kholopov (Nauka Publishing House, Moscow)

Misselt, K. A., and Shafter, A. W., 1995, {\it Astron. J.}, in press

Natsvlishvili, R. Sh., 1984, {\it Inf. Bull. Var. Stars}, No.\,2565

Nogami, D., Kato, T., Masuda, S., and Hirata, R., 1995, in preparation

Ohtani, H., Uesugi, A., Tomita, Y., Yoshida, M., Kosugi, G., Noumaru, J.,
Araya, S., Ohta, K., 1992, {\it Memoirs of the Faculty of Science,
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Osaki, Y., 1995, {\it Publ. Astron. Soc. in Japan}, in press

Robertson, J. W., Honeycutt, R. K., and Turner, G. W., 1995,
          {\it Publ. Astron. Soc. Pacific}, submitted

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

Wolf, M., and Wolf, G., 1906, {\it Astron. Nachr.}, {\bf 171}, 77

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