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tokens: ACKNOWLEDGEMENTS We thank the anonymous referee for the useful comments that allowed us to improve this paper. We also thank P. Bergeron for providing cooling tracks for WDs with mixed atmospheres. The work is based on observations made with the Gran Telescopio Canarias (GTC), installed at the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias, in the island of La Palma. iraf is distributed by the National Optical Astronomy Observatory, which is operated by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the National Science Foundation. This work has made use of data from the European Space Agency (ESA) mission Gaia ( https://www.cosmos.esa.int/gaia ), processed by the Gaia Data Processing and Analysis Consortium (DPAC; https://www.cosmos.esa.int/web/gaia/dpac/consortium ). Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement. This work also used public data from the North American Nanohertz Observatory for Gravitational Waves (NANOGrav; http://nanograv.org ), which is designated as a ‘Physics Frontiers Center’ by the National Science Foundation (award # 1430284). DAZ thanks Pirinem School of Theoretical Physics for hospitality. The work of AYuK, AVK, and DAZ was supported by the Russian Foundation for Basic Research, project no. 18-32-20170 mol_a_ved. The work of SVZ was supported by PAPIIT grants IN-100617 and IN-102120.