Determination of Photospheric Helium Abundances for the Hot White Dwarfs LB 1663 and CD -38 degrees 10980 from EXOSAT Soft X-Ray Photometry

The photospheric soft X-ray emission of hot (T_e> 20,000 K) DA white dwarfs is very sensitive to the composition of the stars' atmospheres. Very small amounts of helium in a hydrogen-dominated atmosphere, amounts which are difficult or impossible to measure at UV and optical wavelengths, can be detected in the soft X-ray band. In this paper we present a measurement of the photospheric helium abundance of LB 1663 and CD - 38°10980, based on three-color soft X-ray photometry obtained with the low-energy imaging telescopes on the EXOSAT observatory, using constraints on T_e and absolute flux levels derived from optical and UV observations. We find for CD -38°10980 that n(He)/n(H) < 5 x 10^-6, almost independent of the other spectral parameters. For LB 1663 we find n(He)/n(H) = (1-2) x 10^-4, in agreement with, but considerably more accurate than, an earlier result obtained with the Einstein IPC.