1999MNRAS.302..731D

Using our extensive ROSAT X-ray observations of M33, we confirm a 3.45-d eclipse period for the Einstein source X-7 proposed by Larson & Schulman and discover evidence for a 0.31-s pulse period. The orbital period, pulse period and observed X-ray luminosity are remarkably similar to those of the Small Magellanic Cloud high-mass X-ray binary SMC X-1. We therefore suggest that M33 X-7 is a neutron star high-mass X-ray binary with a 15–40 M☉ O/B companion and a binary separation of 25–33 R☉ if the companion is almost filling its Roche lobe.