  Recent studies of massive binaries with putative black hole companions have
uncovered a phase of binary evolution that has not been observed before,
featuring a bloated stripped star that very recently ceased transferring mass
to a main-sequence companion. In this study, we focus on the candidate system
VFTS 291, a binary with an orbital period of 108 d and a high semi-amplitude
velocity ($K_1=93.7\pm0.2$ km s$^{-1}$). Through our analysis of the
disentangled spectra of the two components, together with dynamical and
evolutionary arguments, we identify a narrow-lined star of ~1.5-2.5 $M_\odot$
dominating the spectrum, and an early B-type main-sequence companion of
$13.2\pm1.5$ $M_\odot$. The low mass of the narrow-lined star, and the high
mass ratio, suggest that VFTS 291 is a post-mass-transfer system, with the
narrow-lined star being bloated and stripped of its hydrogen-rich envelope,
sharing many similarities with other recently discovered stripped stars. Our
finding is supported by our detailed binary evolution models, which indicate
that the system can be well explained by an initial configuration consisting of
an 8.1 $M_\odot$ primary with an 8 $M_\odot$ companion in a 7 d orbital period.
While some open questions remain, particularly concerning the surface helium
enrichment of the stripped star and the rotational velocity of the companion,
we expect that high-resolution spectroscopy may help reconcile our estimates
with theory. Our study highlights the importance of multi-epoch spectroscopic
surveys to identify and characterize binary interaction products, and provides
important insights into the evolution of massive binary stars.
