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.

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. 
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.

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]. 

Var73 Dra periodogram (2002, Dec 10/11 observations)

Var73 Dra phase diagram (2002, Dec 10/11 observations)





Copyright © 2002 - Tonny Vanmunster.