A selection of 17 stars in the Sloan Digital Sky Survey , previously identified as DC class white dwarfs ( WDs ) , has been reported to show spectra very close to blackbody radiation in the wavelength range from ultraviolet to infrared .
Due to the absence of lines and other details in their spectra , the surface gravity of these objects has not been previously well constrained and their effective temperatures have been determined by fits to the continuum spectrum using pure helium atmosphere models .
We compute model atmospheres with pure helium and H/He mixtures and use Gaia DR2 parallaxes available for 16 out of the 17 selected stars to analyze their physical properties .
We find that the atmospheres of the selected stars are very probably contaminated with a trace amount of hydrogen as -6 \leq \log { \left ( N _ { H } / N _ { He } \right ) } \leq - 5.4 .
For the 16 stars with Gaia parallaxes , we calculate a mean stellar mass 0.606 \pm 0.076 { M } _ { \odot } , which represents typical mass values and surface gravities ( 7.8 < \log g < 8.3 ) for WDs .