2018A&A...616L..17S

We report the detection of X-ray pulsations at 2.1ms from the known X-ray burster IGR J17379-3747 using XMM-Newton. The coherent signal shows a clear Doppler modulation from which we estimate an orbital period of ∼1.9h and a projected semi-major axis of ∼8lt-ms. Taking into account the lack of eclipses (inclination angle of <75°) and assuming a neutron star mass of 1.4M_☉, we have estimated a minimum companion star of ∼0.06M_☉. Considerations on the probability distribution of the binary inclination angle make the hypothesis of a main-sequence companion star less likely. On the other hand, the close correspondence with the orbital parameters of the accreting millisecond pulsar SAX J1808.4-3658 suggests the presence of a bloated brown dwarf. The energy spectrum of the source is well described by a soft disk black-body component (kT∼0.45keV) plus a Comptonisation spectrum with photon index ∼1.9. No sign of emission lines or reflection components are significantly detected. Finally, combining the source ephemerides estimated from the observed outbursts, we obtained a first constraint on the long-term orbital evolution of the order of P_orb=(-2.5±2.3)x10^–12s/s.