The pre-He white dwarf in the post-mass transfer binary EL CVn

EL CVn is the prototype of a class of eclipsing binaries that consist of an A- or F-type main-sequence star and a hot, low-mass, pre-He white dwarf (pre-He WD), the stripped down remains of the former mass donor. Here we present the first direct detection and characterization of the spectrum of the pre-He WD in EL CVn that was made possible through far-UV spectroscopy with the Hubble Space Telescope Cosmic Origins Spectrograph. These spectra straddle the wavelength range where flux dominance shifts from the pre-He WD to the A star. Radial velocities of both components were measured from the far-UV spectra and new optical spectra from the Apache Point Observatory Astrophysical Research Consortium Echelle Spectrograph. We also obtained fast cadence photometry of the eclipses with the pt5m telescope at the Roque de los Muchachos Observatory. A combined analysis of the velocities and light curve yields the component masses and radii. We applied a Doppler tomography algorithm to reconstruct the individual spectra, and we compared these to models to estimate the effective temperatures. The pre-He WD has low mass (0.176 ± 0.004 M ⊙), is small (0.284 ± 0.003 R ⊙), and is relatively hot (11,800 ± 400 K), and these parameters are approximately consistent with predictions for a star stripped through stable mass transfer. The spectral lines of the pre-He WD show that its atmosphere is H-rich, He-depleted, and metal-poor, probably as the result of elemental diffusion that has occurred since mass transfer ceased.