Two-colour photometry of the binary planetary nebula nuclei UU Sagitte and V477 Lyrae: oversized secondaries in post-common-envelope binaries

TL;DR

This study presents detailed photometric analysis of two binary planetary nebula nuclei, revealing oversized secondary stars likely due to thermal disequilibrium or magnetic activity, and estimates their evolutionary parameters.

Contribution

It provides new V and R photometry, combined with radial velocity analysis, to determine physical parameters and evolutionary states of the binary nuclei UU Sge and V477 Lyr, highlighting oversized secondaries.

Findings

Secondary stars are larger than main-sequence counterparts.

Secondary components are out of thermal equilibrium.

Magnetic activity may explain radius expansion in some systems.

Abstract

We present new V and R-passband CCD photometry of UU Sge and V477 Lyr, the eclipsing binary nuclei of the planetary nebulae Abell 63 and Abell 46, respectively. We have performed a simultaneous analysis of VR light-curves and estimated the effective temperatures for the primary and secondary stars to be 78 000 $\pm$ 3000 and 6136 $\pm$ 240 K for UU Sge, 49 500 $\pm$ 4500 and 3874 $\pm$ 350 K for V477 Lyr. We have also reanalysed the previously measured radial velocities and combined the results with those obtained from the analysis of the light curves to derive absolute parameters of the components. The secondary stars have larger radii than expected from their main--sequence counterparts at the same masses. We have determined the post--common envelope ages and the thermal time scales of the systems and examined the possible reasons of expanded radius of the secondary components, together with some selected post-common envelope binaries. We conclude that the secondary components of the nuclei of the planetary nebulae are still out of thermal equilibrium along with two post-common envelope systems: HS 1136+6646 and RE 1016-053. For other systems, magnetic activity has been suggested as the more plausible reason for their expanded radii. We have also estimated the common--envelope efficiency parameters of UU Sge and V477 Lyr.