Conclusions

Although the 'ensemble' method is quite appealing at first glance, and although its first application by [Horne & Stiening (1985)] met success, its application is limited to ``well behaved CVs''. This term, however, is almost a contradiction in itself. As was pointed out by [Welsh et al. (1996)] the sensitivity of the 'ensemble' method to violations of stationarity is a major disadvantage. This does not mean that it cannot yield useful results under carefully chosen and controlled circumstances which, however, for the large majority of CVs will be difficult to realize. In view of the additional difficulties arising from the fact that the light curves available for the present study are all uncalibrated the application of the 'ensemble' method to these data is definitely not suitable. Therefore, the 'single' method is preferred here.

Doubtlessly, the 'single' method also has its drawbacks: It yields rather noisy scatter curves unless the number of light curves is exceedingly high, and it is insensitive to flickering on time scales longer than those of the remaining variations in the smoothed light curves. However, as has been shown by [Bruch (1996)], [Bruch et al. (2000)] and will be shown in Sect. 4 of the present paper, it is capable to produce useful scatter curves of the flickering even for the most unstationary CVs.