Discussion

Recent works ([Robinson et al. (1995)]; [Baptista et al. (1995)]) have exploited the ultraviolet capabilities of the Hubble Space Telescope to observe and map eclipsing CVs. Our investigation bears on these studies in several direct ways.

Eclipse maps, which show the surface brightness of the disk, need to be interpreted very carefully in view of the temperature-dependent (hence radius-dependent) dimming that limb darkening produces: a brightness temperature is not an effective temperature, and it is possible to infer that a disk is not in a steady state when in fact it is, or vice versa. If limb darkening is neglected, the temperature scale may be altered, and the mass transfer rate inferred incorrectly, even if no distortion of the log(R) - log T shape occurs.

Other diagnostic diagrams derived from eclipse maps, such as the ultraviolet-red ``color-magnitude'' diagram for the UX UMa disk, which was used by [Baptista et al. (1995)] to infer a distance to that system, are also susceptible to misinterpretation if limb darkening is neglected. Here the effect is that both bandpasses are affected by limb darkening, by different amounts. Each point in the diagram will be shifted along a trajectory that may or may not be parallel to the locus of the points. If the shift is not parallel, the inferred distance will be in error.

A third effect is that of differential limb darkening, considered in the context of a flared disk. [Robinson et al. (1995)] have postulated a flared disk to remove a front-back brightness asymmetry in their map of the Z Cha disk during outburst. If the disk surface is flared, the amount of limb darkening suffered at a particular disk radius will be different on the front and back sides of the disk. The effect likely operates in such a direction as to decrease substantially the amount of flare required to symmetrize the map.

In view of the high quality of the observations, each of these interpretive points merits a quantitative discussion that is beyond the scope of this paper.