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Recent studies of massive binaries with putative black hole companions haveuncovered a phase of binary evolution that has not been observed before,featuring a bloated stripped star that very recently ceased transferring massto a main-sequence companion. In this study, we focus on the candidate systemVFTS 291, a binary with an orbital period of 108 d and a high semi-amplitudevelocity ($K_1=93.7\pm0.2$ km s$^{-1}$). Through our analysis of thedisentangled spectra of the two components, together with dynamical andevolutionary 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 highmass ratio, suggest that VFTS 291 is a post-mass-transfer system, with thenarrow-lined star being bloated and stripped of its hydrogen-rich envelope,sharing many similarities with other recently discovered stripped stars. Ourfinding is supported by our detailed binary evolution models, which indicatethat the system can be well explained by an initial configuration consisting ofan 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 heliumenrichment of the stripped star and the rotational velocity of the companion,we expect that high-resolution spectroscopy may help reconcile our estimateswith theory. Our study highlights the importance of multi-epoch spectroscopicsurveys to identify and characterize binary interaction products, and providesimportant insights into the evolution of massive binary stars.

The B-type Binaries Characterisation Programme II. VFTS 291: A stripped star from a recent mass transfer phase