Hunting Superhumps -- A New Trend of Amateur Variable Star Research

(a poster presented at Padova CV Conference)

Taichi Kato (Kyoto University, Japan)

1. History

In Japan, dwarf novae (DNe) have been one of the most popular targets among amateur variable star observers. The history of observations dates back to 1930's, when the pioneering members of the OAA (Oriental Astronomical Association) were intensively following the light curves of faint and poorly studied dwarf novae. There work lead to identification of the outburst cycle lengths and their characteristics of large number of dwarf novae. It should be noted that these works were completed well before the famous monograph by J. Grasby. Most of the data are now accesible through machine-readable compilation(*) by the VSOLJ (Variable Star Observers League in Japan). (* The database manager: Makoto Watanabe, e-mail:

After the dark age of World War II and the postwar era, there arose a new trend of variable star research, which was largely affected by the methods of the AAVSO. In addition to this, there appeared a new group of photographic observers under instruction of M. Huruhata (Tokyo Astronomical Observatory). Since 1970's, they started monitoring of faint and poorly understood dwarf novae down to 16-th or 17-th magnitude. Current amateur CV observers in Japan have been more or less affected by the activity of this group.

A turning point came in 1984, when the leading amateurs began realizing "What is the nature of the objects we are observing?". Quite fortunately advised by the theoretical progress such as discovery of the tidal instability by Whitehurst and the mechanism of SU UMa-phenomenon by Osaki, and also stimulated by discovery of superhumps by professional high-speed photometry, some of Japanese amatuers began highly inclined to CVs, esp. the mystery of SU UMa-type dwarf novae.

2. Early Results by VSOLJ Members

After successful time-resolved photographic observation of eclipses IP Peg during outbursts in 1986, the first successful observation of superhumps was achieved in early 1987. The object was famous T Leo.

Encouraged by this success, the VSOLJ members began systematic search for superhumps photographically and visually. The targets were UV Per, SU UMa, AW Gem, AQ Eri, CY UMa, FO And and so on. Some of the results have been published in VSOLJ Variable Star Bulletin.

3. New Stage of Superhump Research

Since 1989, the network communication began prevailing throughout Japan. This was just when the author began CCD observations using the telescope at the university. Coupled with international communication with amateurs enabled us to exchange alerts of rare outbursts of dwarf novae, most of which are "TOADs" as proposed by Howell et al., and enabled the precise CCD photometry at the very early stage of the outburst.

This international cooperation has led us to discover not only superhumps in a large number of suspected dwarf novae, but also early-stage transient phenomena like super-QPOs. The results include establishing superhumps in dwarf novae with extremely rare outbursts like EF Peg, V1251 Cyg, and in a new WZ Sge-type dwarf nova HV Vir. Recent spread of large-aperture telescopes and CCDs among amateurs have also led to discover very faint SU UMa stars like V344 Lyr and KV And, either of them scarcely reach 14-th magnitude even at superoutburst maximum. Very deep visual monitoring by Belgian observers has led to rediscovery of LL And, which was subsequently identified with an SU UMa star with the shortest established superhump period.

4. A New Trend of Amatuer CCD Observations

Very recently, M. Iida, an amateur observer of VSOLJ, has equipped an ST-6 CCD camera to his 16-cm reflector. With the help of data processing software developed by himself, he has attained a limiting magnitude of 18 to 19 under ideal condition. To summarize this poster paper, I would like to intorduce his most recent work "Discovery of Superhumps in Dwarf Nova BC Ursae Majoris" and our follow-up observations.

Discovery of Superhumps in Dwarf Nova BC Ursae Majoris

The light curve constructed from visual and CCD observations from European colleagues and VSOLJ members is shown in figure 1. The detection of the outburst was done on Apr. 28.476, 1994 by Iida. The light curve indicates that the outburst must have started between Apr. 26 and 28. The present outburst reached mv=12.5 at maximum, and lasted 11 +/- 1 days.

Figure 2 shows the result of CCD observations by Iida. Superhumps with an amplitude of 0.3 magnitude are clearly seen, which demonstrates for the first time in the world that the object really belongs to a member of SU UMa stars. Period analysis using PDM gives a best estimate of the period of 0.0619 day.

Figure 3 shows the result of follow-up observation on May 2 performed by Kunjaya at Kyoto University. The observation confirmed the reality of superhumps discovered by Iida. Late superhumps were also observed on May 8 and 9 (figure 4), and quiescent photometry has been obtained. The analysis is now under way, but the prelimiary result of reduction already demonstrates that BC UMa, a typical TOAD, belongs to a group of shortest orbital period dwarf novae.

Finally we would like to emphasize that the present observation at the same time clearly demonstrates a new role of small telescopes in CV research, which has long been believed to require large telescopes with excellent detectors.

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