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 severa l primary minima. Photometry with high signal-to-noise ratio and high time resolution shows that the eclipse is total and the duration of th e flat-bottomed portion of the minimum is similar to 14 min. At this p oint in the light curve, the observed flux must originate from the ave rted hemisphere of the cool secondary star without contamination from the hot, luminous primary component. The observed (V - I)(c) colour in dex 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 coo l star is compared with that predicted from model atmospheres, good ag reement can be obtained with a model generated using T-eff = 6250 +/- 250 K and log g = 4.5. The astrophysical data derived using this tempe rature show little variation from those found by Pollacco and Bell. Co nsistency with post-AGB evolution tracks appropriate to the mass of th e primary component can only be obtained if a limb-brightened model fo r the cool star is invoked. Using the distance of 2.4 +/- 0.4 kpc deri ved 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, l ow-density planetary nebulae and is more closely related to the amount of material expelled during a large nova outburst.