The contact phase expected to precede the coalescence of two massive stars is poorly characterized due to the paucity of observational constraints .
Here we report on the discovery of VFTS 352 , an O-type binary in the 30 Doradus region , as the most massive and earliest spectral type overcontact system known to date .
We derived the 3D geometry of the system , its orbital period P _ { orb } = 1.1241452 ( 4 ) d , components ’ effective temperatures – T _ { 1 } = 42 540 \pm 280 K and T _ { 2 } = 41 120 \pm 290 K – and dynamical masses – M _ { 1 } = 28.63 \pm 0.30 ~ { } M _ { \odot } and M _ { 2 } = 28.85 \pm 0.30 ~ { } M _ { \odot } .
Compared to single-star evolutionary models , the VFTS 352 components are too hot for their dynamical masses by about 2700 and 1100 K , respectively .
These results can be explained naturally as a result of enhanced mixing , theoretically predicted to occur in very short-period tidally-locked systems .
The VFTS 352 components are two of the best candidates identified so far to undergo this so-called chemically homogeneous evolution .
The future of VFTS 352 is uncertain .
If the two stars merge , a very rapidly rotating star will be produced .
Instead , if the stars continue to evolve homogeneously and keep shrinking within their Roche Lobes , coalescence can be avoided .
In this case , tides may counteract the spin down by winds such that the VFTS 352 components may , at the end of their life , fulfill the requirements for long gamma-ray burst ( GRB ) progenitors in the collapsar scenario .
Independently of whether the VFTS 352 components become GRB progenitors , this scenario makes VFTS 352 interesting as a progenitor of a black hole binary , hence as a potential gravitational wave source through black hole-black hole merger .