BELGIAN ASTRONOMICAL SOCIETY V.V.S. -  Working Group Variable Stars
Cataclysmic Variables Circular No.157              1997, October 20
Ed.: T. Vanmunster, Walhostraat 1A, 3401 Landen, BELGIUM
     Internet: tvanmuns@innet.be                  TEL. 32-11-831504
     CVC Web Page : http://www.astro.columbia.edu/~cba/cvc
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SW UMa [UGSU, 10.6v - 16.5v]
============================

Gene Hanson, Az and Bjorn Granslo, Norway independently report their
visual detection of an outburst of this very interesting UGSU-type 
dwarf nova. 

1997 Oct 18.508 UT, [14.6  (G. Hanson, seq. AAVSO);
     Oct 19.17  UT, [13.0  (P. Schmeer, seq. AAVSO);
     Oct 19.490 UT,  11.1  (G. Hanson);
     Oct 19.969 UT,  10.4  (B. Granslo, seq. Tycho);
     Oct 20.353 UT,  10.5  (G. Hanson);


SW UMa was last seen in outburst on 1996, April 11 by Lasse
Jensen, Denmark [CVC 90]. This was a very well covered super-
outburst [CVC 93], both visually and by CCD. 

Orbital and superhump periods of SW UMa are well determined. The
following values are given in [1] : orbital period 1.36356 h, super-
hump period 1.3999 h.

This object provides a lot of interesting observing opportunities,
both for amateurs and professionals :

- Superhumps in SW UMa can easily be detected and followed visually.
  Bjorn Granslo, Norway has presented an interesting account of his
  attempts during the March 1990 superoutburst in [2].

- Astronomers with CCD equipment should look for QPOs (quasi-periodic 
  oscillations). QPOs are oscillations of short coherence lengths, and
  are a widely observed phenomenon in cataclysmic binaries. During the 
  1992 superoutburst of SW UMa, Taichi Kato of Ouda Observatory, Kyoto 
  University, Japan, detected QPOs with gigantic amplitudes (~20%). 
  This type of QPOs are called "super-QPOs". Superimposed with the 
  superhumps, they observed QPOs with a mean period of 6.1 minutes and 
  an unprecedented large amplitude of 0.2 mag. [3].


References :
------------

[1] Cataclysmic Variable Stars, B. Warner, Cambridge Astrophysics
    Series, No. 28, Cambridge University Press, 1995.
[2] Variable Star Research: An International Perspective, ed. J.R.
    Percy et al., Cambridge University Press, 1992.
[3] Kato et al. 1992, PASJ 44, L215


Tonny Vanmunster