DIRECT OPTICAL OBSERVATIONS OF THE SECONDARY COMPONENT OF UU SAGITTAE

CCD photometry and spectroscopy are presented for the eclipsing binary central star of the planetary nebula Abell 63, obtained during several primary minima. Photometry with high signal-to-noise ratio and high time resolution shows that the eclipse is total and the duration of the flat-bottomed portion of the minimum is similar to 14 min. At this point in the light curve, the observed flux must originate from the averted hemisphere of the cool secondary star without contamination from the hot, luminous primary component. The observed (V - I)(c) colour index at phase 0.0 is 0.96 +/- 0.09. If this star can be thought of as a main-sequence star then (V - I)(o) = 0.34 +/- 0.25, implying T-eff = 7200(-1300)(+1500) K. When the dereddened flux distribution of the cool star is compared with that predicted from model atmospheres, good agreement can be obtained with a model generated using T-eff = 6250 +/- 250 K and log g = 4.5. The astrophysical data derived using this temperature show little variation from those found by Pollacco and Bell. Consistency with post-AGB evolution tracks appropriate to the mass of the primary component can only be obtained if a limb-brightened model for the cool star is invoked. Using the distance of 2.4 +/- 0.4 kpc derived from our analysis, the ionized mass of Abell 63 is found to be at least an order of magnitude less than that expected for typical old, low-density planetary nebulae and is more closely related to the amount of material expelled during a large nova outburst.