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tokens: part. The short-wavelength region contains the strong Balmer lines of H γ , H δ , and H ε , while the long-wavelength section only contains H β and weaker lines of He i λλ 4387, 4471, 4713, 4921. The difference in the CCF strength for the two regions suggests that the H Balmer lines are the dominant contributor to the CCF amplitude. In the similar, lower-mass binary, EL CVn, evidence suggests the presence of a thick hydrogen envelope even after extreme mass loss (Maxted et al. 2014 ), and the spectrum of the companion in EL CVn shows strong H Balmer lines with narrow cores (Wang et al. 2020 ), consistent with their appearance in the model CCF template spectra for solar abundances that we use here. Thus, the companion of Regulus probably has a photosphere with a significant hydrogen abundance. 4. Discussion The cross-correlation analysis of the large set of high-S/N spectra from ESPaDOnS/NARVAL reveals a weak signal that has all the properties expected for the spectral signature of the pre-white dwarf companion of Regulus. The CCF peak only appears using model spectra for a hot object, and it attains the strongest and narrowest morphology for a secondary semiamplitude that is consistent with the predicted extreme mass ratio and low mass of the pre-white dwarf remnant. The narrow appearance of the peak agrees with expectations of a low projected rotational velocity for synchronous rotation that is predicted in models of binary evolution. The mass and radius implied by the observations lead to a predicted small gravitational redshift that is consistent with the measured peak velocity. Taken together, these results indicate that the CCF peak marks the first detection of the spectral features of the pre-white dwarf companion of Regulus. The stellar masses we derive from the CCF analysis are similar to those proposed in earlier work (Gies et al. 2008 ). Che et al. ( 2011 ) used the stellar temperature and luminosity and the Yale–Yonsei evolutionary tracks to find a primary mass of M 1 = (4.15 ± 0.06) M ⊙ . This agrees within uncertainties with our result, M 1 = (3.7 ± 1.4) M ⊙ . The pre-white dwarf star has a low mass, , which represents the mass of the surviving core after completion of mass transfer. There is a close relationship between core mass and the properties of the former, Roche-filling red giant, and this implies that the core mass is related to the orbital period of the system. Rappaport et al. ( 2009 ) describe how the relation applies to the Regulus system and how the orbital period leads to a predicted pre-white dwarf mass of M 2 = (0.28 ± 0.05) M ⊙ , and this estimate agrees within uncertainties with the measured mass. The evolution of the stellar core after envelope removal was investigated by Istrate et al. ( 2016 ) using MESA models to study the internal structure and tracks in the plane. We show in Figure 8 one example for a pre-white dwarf with an He core and a mass of 0.305 M ⊙ from the basic set they present, i.e.,