J0526+5934: A peculiar ultra-short-period double white dwarf

Context. Ultra-short-period compact binaries are important sources of gravitational waves. The class of short-period compact binaries includes, for example, the progenitors of type Ia supernovae and the progenitors of merger episodes that may lead to massive and magnetic single white dwarfs. J0526+5934 is one such example: it is an unresolved compact binary star with an orbital period of 20.5 min. Aims. The visible component of J0526+5934 was recently claimed to be a hot sub-dwarf star with a CO white dwarf companion. Our aim is to provide strong observational and theoretical evidence that the primary star is instead an extremely low-mass white dwarf, although the hot sub-dwarf nature cannot be completely ruled out. Methods. We analysed optical spectra together with time-series photometry of the visible component of J0526+5934 to constrain its orbital and stellar parameters. We also employed evolutionary sequences for low-mass white dwarfs to derive independent values of the primary mass. Results. From the analysis of our observational data, we find a stellar mass for the primary star in J0526+5934 of 0.26 +/- 0.05 M-circle dot, which perfectly matches the 0.237 +/- 0.035 M-circle dot independent measurement we derive from the theoretical evolutionary models. This value is considerably lower than the theoretically expected and generally observed mass range for hot sub-dwarf stars, but falls well within the mass limit values of extremely low-mass white dwarfs. Conclusions. We conclude J0526+5934 is the sixth ultra-short-period detached double white dwarf currently known.