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

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 +/- 0.2 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 similar to 1.5-2.5 M-circle dot dominating the spectrum, and an early B-type main-sequence companion of 13.2 +/- 1.5 M-circle dot. 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 disco v ered 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-circle dot primary with an 8 M-circle dot 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 surv e ys to identify and characterize binary interaction products, and provides important insights into the evolution of massive binary stars.