Rediscussion of eclipsing binaries. Paper XXV. The chemically-peculiar system AR Aurigae

Abstract: AR Aur is a detached eclipsing binary containing two late-B stars which are chemically peculiar, on a circular orbit of period 4.135 d. The primary is a HgMn star which shows temporal changes in its chemical abundances and spectral line profiles, whilst the secondary is a likely weak Am star. Published analyses of the system have used spectroscopic light ratios to constrain the eclipse models and found that the secondary star is larger than the primary. This unexpected outcome has been taken as an indication that the system is young and the secondary has yet to reach the main sequence. In this work we present the first analysis of the light curve of the system obtained by the Transiting Exoplanet Survey Satellite (TESS), whose quality allows us to avoid using a spectroscopic light ratio to constrain the solution. When combined with literature spectroscopic results we obtain highly precise masses of 2.544 +/- 0.009 Msun and 2.358 +/- 0.009 Msun, and radii of 1.843 +/- 0.002 Rsun and 1.766 +/- 0.003 Rsun. The light ratio is inconsistent with spectroscopic determinations, confirming the suggestion of Takeda (2025) that spectroscopic light ratios of the system are unreliable due the chemical peculiarity of the stars. The properties of the system are matched by theoretical predictions for a slightly super-solar metallicity and an age of 33 +/- 3 Myr: both components are young main-sequence stars.