Extreme mass ratios and fast rotation in three massive binaries

The origin of rapid rotation in massive stars remains debated, although
binary interactions are now often advocated as a cause. However, the broad and
shallow lines in the spectra of fast rotators make direct detection of binarity
difficult. In this paper, we report on the discovery and analysis of
multiplicity for three fast-rotating massive stars: HD25631 (B3V), HD191495
(B0V), and HD46485 (O7V). They display strikingly similar TESS light curves,
with two narrow eclipses superimposed on a sinusoidal variation due to
reflection effects. We complement these photometric data by spectroscopy from
various instruments (X-Shooter, Espadons, FUSE...), to further constrain the
nature of these systems. The detailed analyses of these data demonstrates that
the companions of the massive OB stars have low masses (~1Msol) with rather
large radii (2-4 Rsol) and low temperatures (<15 kK). These companions display
no UV signature, which would exclude a hot subdwarf nature, but disentangling
of the large set of X-Shooter spectra of HD25631 revealed the typical signature
of chromospheric activity in the companion's spectrum. In addition, despite the
short orbital periods (P=3-7d), the fast-rotating OB-stars still display
non-synchronized rotation and all systems appear young (<20Myr). This suggests
that, as in a few other cases, these massive stars are paired in those systems
with non-degenerate, low-mass PMS companions, implying that fast rotation would
not be a consequence of a past binary interactions in their case.