Context : BD +60 ^ { \circ } 73 is the optical counterpart of the X-ray source IGR J00370+6122 , a probable accretion-powered X-ray pulsar .
The X-ray light curve of this binary system shows clear periodicity at 15.7 d , which has been interpreted as repeated outbursts around the periastron of an eccentric orbit .

Aims : We aim to characterise the binary system IGR J00370+6122 by deriving its orbital and physical parameters .

Methods : We obtained high-resolution spectra of BD +60 ^ { \circ } 73 at different epochs .
We used the fastwind code to generate a stellar atmosphere model to fit the observed spectrum and obtain physical magnitudes .
The synthetic spectrum was used as a template for cross-correlation with the observed spectra to measure radial velocities .
The radial velocity curve provided an orbital solution for the system .
We also analysed the RXTE /ASM and Swift /BAT light curves to confirm the stability of the periodicity .

Results : BD +60 ^ { \circ } 73 is a BN0.7 Ib low-luminosity supergiant located at a distance \sim 3.1 kpc , in the Cas OB4 association .
We derive T _ { { eff } } = 24 000 K and \log g _ { c } = 3.0 , and chemical abundances consistent with a moderately high level of evolution .
The spectroscopic and evolutionary masses are consistent at the 1- \sigma level with a mass M _ { * } \approx 15 M _ { \sun } .
The recurrence time of the X-ray flares is the orbital period of the system .
The neutron star is in a high-eccentricity ( e = 0.56 \pm 0.07 ) orbit , and the X-ray emission is strongly peaked around orbital phase \phi = 0.2 , though the observations are consistent with some level of X-ray activity happening at all orbital phases .

Conclusions : The X-ray behaviour of IGR J00370+6122 is reminiscent of “ intermediate ” supergiant X-ray transients , though its peak luminosity is rather low .
The orbit is somewhat wider than those of classical persistent supergiant X-ray binaries , which when combined with the low luminosity of the mass donor , explains the low X-ray luminosity .
IGR J00370+6122 will very likely evolve towards a persistent supergiant system , highlighting the evolutionary connection between different classes of wind-accreting X-ray sources .