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tokens: ACKNOWLEDGEMENTS The authors thank the anonymous referee for useful comments which allowed them to improve the manuscript, Emmanuel Fonseca (on behalf of the NANOGrav collaboration) for providing probabilities of the Shapiro-delay parameters from the NANOGrav 9-yr data set and Maxim Voronkov for helpful discussions. The work of AVK and DAZ (Section 3 ) was supported by the Russian Foundation for Basic Research, project No. 18-32-00781 mol_a. AYuK and SVZ acknowledge PAPIIT grant IN-100617 for resources provided towards this research. The manuscript is based on observations made with the GTC, installed in 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 research has made use of the USNOFS Image and Catalogue Archive operated by the United States Naval Observatory, Flagstaff Station ( http://www.nofs.navy.mil/data/fchpix/ ). Funding for the SDSS IV has been provided by the Alfred P. Sloan Foundation, the U.S. Department of Energy Office of Science, and the Participating Institutions. SDSS-IV acknowledges support and resources from the Center for High-Performance Computing at the University of Utah. The SDSS web site is www.sdss.org . The authors used WD colour tables available at http://www.astro.umontreal.ca/bergeron/CoolingModels .