Revisiting the post-common-envelope binaries HZ 9, KV Vel and Abell 65

Abstract

ABSTRACT New time-resolved photometric observations of three close but detached white dwarf/red dwarf binaries have enabled an in-depth analysis of their properties, combining new data with known information. For HZ 9, time-resolved photometric data have not been published previously. Our data reveal sinusoidal variations, with the orbital period showing unusual phase shifts between expected and observed light maximum epochs when combining our photometry with the precise radial velocity ephemeris. There are also variations of the photometric amplitude between 0.04 and 0.11 mag. Phase shifts and amplitude variations could be the result of a relatively strong magnetic activity of the secondary M star component, which seems to superimpose the usual reflection effect. This effect is dominant in the two remaining targets of our study. We improve the accuracy of the photometric ephemeris of KV Vel, complementing published data with light curves from the All-Sky Automated Survey (ASAS) and ASAS-SN and with our own observations. Finally, we also add new data for the central star of the planetary nebula Abell 65, complementing and confirming its previously published photometric ephemeris.