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The stellar population of the 30 Doradus star-forming region in the LargeMagellanic Cloud contains a subset of apparently single, rapidly rotatingO-type stars. The physical processes leading to the formation of this cohortare currently uncertain. One member of this group, the late O-type star VFTS399, is found to be unexpectedly X-ray bright for its bolometric luminosity -in this study we aim to determine its physical nature and the cause of thisbehaviour. We find VFTS 399 to be an aperiodic photometric variable with anapparent near-IR excess. Its optical spectrum demonstrates complex emissionprofiles in the lower Balmer series and select HeI lines - taken together thesesuggest an OeBe classification. The highly variable X-ray luminosity is toogreat to be produced by a single star, while the hard, non-thermal naturesuggests the presence of an accreting relativistic companion. Finally, thedetection of periodic modulation of the X-ray lightcurve is most naturallyexplained under the assumption that the accretor is a neutron star. VFTS 399appears to be the first high-mass X-ray binary identified within 30 Dor.Comparison of the current properties of VFTS 399 to binary-evolution modelssuggests a progenitor mass in excess of 25Msun for the putative neutron star,which may host a magnetic field comparable in strength to those of magnetars.VFTS 399 is now the second member of the cohort of rapidly rotating `single'O-type stars in 30 Dor to show evidence of binary interaction resulting inspin-up, suggesting that this may be a viable evolutionary pathway for theformation of a subset of this stellar population.

The VLT-FLAMES Tarantula survey XX. The nature of the X-ray bright emission line star VFTS 399