The physical properties of post-mass-transfer binaries

Aims. We present and analyse the detailed physical properties of six binary stellar systems, originally proposed as possible star-black hole binaries on the basis of radial velocities from Gaia's third data release, but soon recognised as likely post-mass-transfer binary systems with stripped companions. Methods. We used multi-epoch high-resolution FEROS spectra and spectral disentangling paired with stellar templates to derive effective temperatures, $T_\mathrm{eff}$; stellar radii, R*; and projected rotational velocities, v$\sin{i}$ for both components in all systems along with the mass ratio, q = $M_\mathrm{accretor}/M_\mathrm{donor}$ and the components' flux ratio as a function of wavelength. Results. Our analysis directly confirms that all systems are post-mass-transfer binaries with two luminous stars, i.e. no black hole companions. Each system contains an A-type accretor component that is rapidly rotating and a cooler very low-mass donor (~ 0.25M$\odot$) that is overluminous. Five of the systems show no trace of any emission lines, implying that there is no current mass transfer, consistent with our inferred radii, in all cases within the Roche volume. The data are generally consistent with stable case AB mass transfer with $β$ (the fraction of mass lost from the accretor) less than 0.7. While the accretor components rotate rapidly, they rotate well below the critical rotation rate, $v_\mathrm{crit}$, even though there must have been enough mass transfer to spin them up to a significant fraction of $v_\mathrm{crit}$, according to theoretical models of angular momentum transfer. As neither magnetic braking nor tidal synchronisation should have been effective in spinning down the stars, our results suggest that either mass accretion does not increase the angular momentum of the accretors to their critical values or the systems never reached these values in the first place.