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Var73 Dra :
first ER UMa type dwarf nova IN the period gap.
Confirmative observations during the December 2002 outburst.
Var73 Dra is an SU
UMa type dwarf nova, discovered by Antipin and
Pavlenko (A&A 391, 565 (2002), located at RA = 20h23m38s.193
and dec = +64o36'26".91 (J2000.0), with a magnitude
range of 15.8p-19.3R. The authors report a
superhump period of 0.0954 d, which is one of the
longest among SU UMa type dwarf novae !
In October 2002,
Var73 Dra was found in superoutburst and studied
quite intensively by the VSNET Collaboration Team.
They determined a refined superhump period of
0.1052(2) d, and classified the object as a likely
"twin" system of NY Ser, an SU UMa-type dwarf nova
in the period gap
(Nogami et al. (1998) 50, L1).
On December 7th,
2002, Var73 Dra was again detected in outburst, by
the Kyoto team [vsnet-alert 7588]. The brightness
was comparable to that of the October 2002
superoutburst. Taichi Kato remarked in that
vsnet-alert message : "If the present
outburst turns out to be a superoutburst, Var73 Dra
may become the first object having a supercycle
length between usual SU UMa stars and ER UMa
stars."
I observed Var73
Dra on 2002, Dec 08/09, under relatively good
atmospheric conditions, with the 0.35-m f/6.3
telescope and ST-7 CCD (unfiltered).
The session lasted for 2.66
hours, and the resulting light curve is shown
at right. Although small scale variations are
clearly present, the light curve does not show
any superhump characteristics. |
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On December
09/10, 2002, the skies were again clear (and
very cold) over Belgium, allowing me to follow
Var73 Dra during 3.21 hours, with the same
equipment as described above. Atmospheric
conditions were relatively good, although
strong winds were present.
The resulting light curve is shown at right.
It once more shows small amplitude light
variations, but no obvious superhump
signature. |
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The long spell of
clear skies (according to Belgian standards)
continued, allowing me to obtain Var73 Dra CCD
observations also on December 10/11, 2002.
This time, sky conditions were very good.
I followed the object for 4.47 hours, which
resulted in the light curve at right.
Superhumps now have clearly emerged,
confirming the superoutburst nature of
the December 2002 outburst. |
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Data
analysis
Over the first 3 nights, I
collected 390 observations of Var73 Dra. During the
first 2 nights (December 08/09 and December 09/10),
the average amplitude of the light modulations was
about 0.10 mag. This increased considerably on
December 10/11, at the moment superhumps started to
appear. The amplitude was now 0.21 mag.
On the basis of the
December 10/11, 2002 observations, and using the PDM
method, I found a superhump period value of 0.1069 +/-
0.002 d (short time basis). The periodogram and phase
diagram are shown below.
Our observations thus confirm
the superhump period value determined by the Kyoto
team during the October 2002 outburst of Var73 Dra
(see above), and the fact that this is a dwarf nova IN
the period gap. In addition, our observations establish
the superoutburst nature of the present outburst,
as was already assumed from a set of CCD observations
obtained by Donn Starkey one night ago (see
vsnet-superoutburst 1761).
More important is the
conclusion that the supercycle of Var73 Dra is around
60 days, similar to ER UMa type dwarf novae. Var73 Dra
therefore is the first ER UMa type dwarf nova IN
the period gap, confirming Taichi Kato's
assumption [vsnet-alert 7588].
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Var73 Dra periodogram (2002, Dec
10/11 observations) |
Var73 Dra phase diagram (2002, Dec
10/11 observations) |
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Copyright ©
2002 - Tonny Vanmunster.
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