UU Sagittae - Eclipsing nucleus of the planetary nebula Abell 63

UU Sge, the central star of the low-surface-brightness planetary nebula Abell 63, is shown to be an eclipsing binary with an orbital period of 0.465 day. Primary minimum is due to total eclipse of the very hot primary star by a cool secondary. The observational material is consistent with a system containing a sdO primary of about 0.9 solar mass and 0.4 solar radius and a dK secondary of approximately 0.7 solar mass and 0.7 solar radius, separated by about 3 solar radii. The hot star heats the hemisphere of the dK star facing it to about 10,000 K. UU Sge may be descended from a common-envelope binary of much longer period, and with cooling of the primary it will probably resemble successively the systems HZ 22, V471 Tau, and PG 1413+01; ultimately it will become a cataclysmic variable. Evidence is assembled indicating that nebula ejection by binary-star processes may not be uncommon.