HT Cas belongs to the SU UMa subclass of CVs. In the prototypical eclipsing members of this group the light curves exhibit a strong orbital hump and the eclipse profile permits to separate the eclipse of the white dwarf and of the hot spot (OY Car: [Wood et al. 1989a]; Z Cha: [Wood et al. 1986]). In this respect HT Cas is rather unstable: An orbital hump is sometimes present; at other epochs no trace of it is visible. It is never as prominent as in OY Car and Z Cha. Similarly, the eclipse profiles may or may not show the typical structure caused by a white dwarf eclipse, followed slightly later by a hot spot eclipse (see e.g. Fig. 8 of Patterson 1981). A representative light curve and the mean of all curves studied here (normalized to a common mean count rate) are shown in Figs. 3a and 3b. There is no significant orbital hump in the mean curve, and the eclipse profile contains at most a remnant of a two-step eclipse (white dwarf and hot spot). Thus, on the average the hot spot has no significant influence on the light curve shape in the present data.
When applying the 'single' method to the HT Cas light curves it was found that due to the very sudden start and end of the white dwarf eclipse ingress and egress and to the short duration of these phases the spline interpolation to the binned light curve (performed as outlined by Bruch 1996) could not follow well the eclipse ingress and egress, causing large residua in the difference curve at these phases. These translated into artificial peaks in the scatter curve. To alleviate this problem additional fiducial points for the spline interpolation were defined interactively at the beginning and end of the steep parts of eclipse ingress and egress. While this significantly reduced the height of the sharp peaks in the scatter curve, it could not remove them completely.
The resulting scatter curve, calculating the scatter in phase intervals of width 0.005 and adopting a step-width of 0.0025 (meaning that neighbouring points in the scatter curve are not independent of each other) is shown in Fig. 3c. Each point in the final scatter curve is the mean value of several individual curves. An error of (``mean error of the mean'') is assigned to each point, where m is the number of individual points contributing to the mean, and sigma is the standard deviation. The representative error bar shown in the upper left corner of Fig. 3c is the average value (= 0.097) of the errors of all data points. The dashed vertical lines are the eclipse contact phases of the white dwarf as measured by [Horne et al. (1991)]. It is seen that - disregarding the artificial peaks occurring during eclipse ingress and egress - the minimum of the scatter curve coincides with the eclipse of the white dwarf. Thus, the flickering light source in HT Cas is well centered on the central body.
Although it may not be very significant in view of the noise in the scatter curve, the scatter eclipse appears to be V-shaped rather than flat-bottomed. If this is true it would mean that the flickering light source is a bit larger in extension than the white dwarf itself. However, it cannot be much larger because otherwise the ingress of the scatter eclipse should start earlier (and end later) than the white dwarf eclipse as the secondary star covers more and more of the region where flickering occurs. This is not seen.
There is no significant enhancement of the scatter during the phase interval in which the canonical hot spot is visible in many CVs. Thus the region of impact of the transferred matter onto the accretion disk appears not to host a flickering light source in HT Cas. The lack of any systematic trend of the scatter which phase (except for the eclipse) is formally confirmed by a Gauss fit to a histogram of the (out-of-eclipse) data points which yields a standard deviation of 0.101, almost identical to the average mean error of the data points of 0.097. This constancy of the scatter (disregarding the eclipse) is different from what is found for e.g. Z Cha (Bruch 1996), V893 Sco (Bruch et al. 2000) and IP Peg (see Sect. 4.3).
The present quantitative results are in agreement with qualitative conclusions of [Patterson (1981)], namely that the flickering in HT Cas originates from regions very close to the white dwarf.