Both components of WD 1242-105 are low-mass white dwarfs, possibly helium-core white dwarfs. WD 1242-105 follows the trend whereby binaries containing low-mass helium-core white dwarfs tend to have shorter orbital periods than binaries containing more massive carbon/oxygen-core white dwarfs. This is because shorter period binaries interact more strongly and experience greater mass loss, resulting in low-mass helium-core white dwarfs.
Debes et al. (2015), “A New Merging Double Degenerate Binary in the Solar Neighborhood”, arXiv:1503.00349 [astro-ph.SR]