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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 ∼1.5–2.5 |$\, \mathrm{M}_\odot$| dominating the spectrum, and an early B-type main-sequence companion of 13.2 ± 1.5 |$\, \mathrm{M}_\odot$|⁠. 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 |$\, \mathrm{M}_\odot$| primary with an 8 |$\, \mathrm{M}_\odot$| 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. [ABSTRACT FROM AUTHOR]

We present results from the B-type binaries characterization (BBC) programme, a multi-epoch spectroscopic study of 88 early B-type binary candidates in the 30 Doradus region of the Large Magellanic Cloud (LMC). From radial-velocity analysis of 29 observational epochs, we confirm the binary status of 64 of our targets, comprising 50 SB1 and 14 SB2 B-type binaries. A further 20 systems (classified as SB1*) show clear signs of periodicity but with more tentative periods. Orbital solutions are presented for these 84 systems, providing the largest homogeneous sample to date of the binary properties of early B-type stars. Our derived orbital-period distribution is generally similar to those for samples of more massive (O-type) binaries in both the LMC and the Galaxy. This similarity with the properties of the more massive O-type binaries is important as early B-type stars are expected to account for the majority of core-collapse supernovae. Differences in the period distributions of the different samples start to increase above 4 d, and are also present between the earliest (B0-0.7) and later-type (B1-2.5) systems within the BBC sample, although further study is required to understand if this is an observational bias or a real physical effect. We have examined the semi-amplitude velocities and orbital periods of our sample to identify potential candidates that could hide compact companions. Comparing with probability distributions of finding black hole companions to OB-type stars from a recent theoretical study, we have found 16 binaries in the higher probability region that warrant further study. [ABSTRACT FROM AUTHOR]

The evolutionary status of Be-type stars remains unclear, with both single-star and binary pathways having been proposed. Here, VFTS spectroscopy of 73 Be-type stars, in the spectral-type range, B0–B3, is analysed to estimate projected rotational velocities, radial velocities, and stellar parameters. They are found to be rotating faster than the corresponding VFTS B-type sample but simulations imply that their projected rotational velocities are inconsistent with them all rotating at near critical velocities. The de-convolution of the projected rotational velocities estimates leads to a mean rotational velocity estimate of 320–350 km s−1, approximately 100 km s−1 larger than that for the corresponding B-type sample. There is a dearth of targets with rotational velocities less than 0.4 of the critical velocity, with a broad distribution reaching up to critical rotation. Our best estimate for the mean or median of the rotational velocity is 0.68 of the critical velocity. Rapidly rotating B-type stars are more numerous than their Be-type counterparts, whilst the observed frequency of Be-type stars identified as binary systems is significantly lower than that for normal B-type stars, consistent with their respective radial-velocity dispersions. The semi-amplitudes for the Be-type binaries are also smaller. Similar results are found for a Small Magellanic Cloud Be-type sample centred on NGC 346 with no significant differences being found between the two samples. These results are compared with the predictions of single and binary stellar evolutionary models for Be-type stars. Assuming that a single mechanism dominated the production of classical Be-type stars, our comparison would favour a binary evolutionary history. [ABSTRACT FROM AUTHOR]

New time-resolved photometric observations of three close but detached white dwarf/red dwarf binaries have enabled an in-depth analysis of their properties, combining new data with known information. For HZ 9, time-resolved photometric data have not been published previously. Our data reveal sinusoidal variations, with the orbital period showing unusual phase shifts between expected and observed light maximum epochs when combining our photometry with the precise radial velocity ephemeris. There are also variations of the photometric amplitude between 0.04 and 0.11 mag. Phase shifts and amplitude variations could be the result of a relatively strong magnetic activity of the secondary M star component, which seems to superimpose the usual reflection effect. This effect is dominant in the two remaining targets of our study. We improve the accuracy of the photometric ephemeris of KV Vel, complementing published data with light curves from the All-Sky Automated Survey (ASAS) and ASAS-SN and with our own observations. Finally, we also add new data for the central star of the planetary nebula Abell 65, complementing and confirming its previously published photometric ephemeris. [ABSTRACT FROM AUTHOR]