Spectral evolution of hot hybrid white dwarfs

II. Photometry

¹ Landessternwarte Heidelberg, Zentrum für Astronomie, Ruprecht-Karls-Universität, Königstuhl 12, 69117 Heidelberg, Germany
² Institut für Astronomie und Astrophysik, Kepler Center for Astro and Particle Physics, Eberhard Karls Universität, Sand 1, 72076 Tübingen, Germany
³ Instituto de Astrofìsica de Canarias, 38205 La Laguna, Tenerife, Spain
⁴ Departamento de Astrofìsica, Universidad de La Laguna, 38206 La Laguna, Tenerife, Spain
⁵ Institut für Physik und Astronomie, Universität Potsdam, Haus 28, Karl-Liebknecht-Str. 24/25, 14476 Potsdam, Germany

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 2 September 2025
Accepted: 29 October 2025

Abstract

We present a photometric analysis of 19 DA and 13 DAO white dwarfs (WDs) with effective temperatures exceeding 60 000 K, building on the spectral analysis reported in the first paper of this two-part study. By examining archival light curves for periodic signals, we identify that four of the 32 objects (13₋₄⁺⁸%) exhibit photometric variability. Spectral energy distribution (SED) fitting allowed us to derive radii, luminosities, and gravity masses, as well as to characterise the infrared excesses observed in six sources. A notable discovery is the identification of a 1.87 d period in the ZTF light curves of WD 1342+443 and weak emission lines in the optical spectra of this star, which strongly indicate an irradiation effect system. Our SED fit indicates the presence of cool dust, which must be located farther from the star, and that any companion with a spectral type earlier than L2.0 would appear in the SED. This leads us to speculate that WD 1342+443 might have an irradiated, sub-stellar companion. We also highlight that we uncovered, for the first time, a 4.23 d photometric period in the well-known, close DA+dM binary WD 0232+035, based on TESS data. We find that the phase and amplitude of the light curve variations are consistent with expectations from an irradiation effect. Intriguingly, we detected an additional, mysterious period at 1.39 days, which is approximately one-third of the orbital period. Moreover, we revisited the longstanding discrepancy between Kiel and gravity masses for the hottest WDs. To address this, we explored fully metal line blanketed model atmospheres as a potential solution, contrasting them with the results from pure-hydrogen and hydrogen-helium models. Our results show that including metal opacities does not resolve the discrepancy – in fact, it slightly deteriorates the agreement. Finally, we reaffirm the previously observed correlation between helium abundance and luminosity.