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BD+6073 is the optical counterpart of the X-ray source IGR J00370+6122, aprobable accretion-powered X-ray pulsar. The X-ray light curve of this binarysystem shows clear periodicity at 15.7 d, which has been interpreted asrepeated outbursts around the periastron of an eccentric orbit. We obtainedhigh-resolution spectra of BD+6073 at different epochs. We used the FASTWindcode to generate a stellar atmosphere model to fit the observed spectrum andobtain physical magnitudes. The synthetic spectrum was used as a template forcross-correlation with the observed spectra to measure radial velocities. Theradial velocity curve provided an orbital solution for the system. We have alsoanalysed the RXTE/ASM and Swift/BAT light curves to confirm the stability ofthe periodicity. BD +6073 is a BN0.7 Ib low-luminosity supergiant located at anapproximate distance of 3.1 kpc, in the CasOB4 association. We deriveTeff=24000 K and log gc=3.0, and chemical abundances consistent with amoderately high level of evolution. The spectroscopic and evolutionary massesare consistent at the 1 sigma level with a mass of 15 solar masses. Therecurrence time of the X-ray flares is the orbital period of the system. The NSis in a high eccentricity (e=0.56) orbit, and the X-ray emission is stronglypeaked around orbital phase 0.2, though the observations are consistent withsome level of X-ray activity happening at all orbital phases. The X-raybehaviour of IGR J00370+6122 is reminiscent of intermediate SFXTs, though itspeak luminosity is rather low. The orbit is somewhat wider than those ofclassical persistent supergiant X-ray binaries, which, combined with the lowluminosity of the mass donor, explains the low X-ray luminosity. IGRJ00370+6122 will likely evolve towards a persistent supergiant system,highlighting the evolutionary connection between different classes ofwind-accreting X-ray sources.

Astrophysical parameters and orbital solution of the peculiar X-ray transient IGR J00370+6122