  NGC 7793 P13 is a variable (luminosity range ~100) ultraluminous X-ray source
(ULX) proposed to host a stellar-mass black hole of less than 15 M$_{\odot}$ in
a binary system with orbital period of 64 d and a 18-23 M$_{\odot}$ B9Ia
companion. Within the EXTraS project we discovered pulsations at a period of
~0.42 s in two XMM-Newton observations of NGC 7793 P13, during which the source
was detected at $L_{\mathrm{X}}\sim2.1\times10^{39}$ and $5\times10^{39}$ erg
s$^{-1}$ (0.3-10 keV band). These findings unambiguously demonstrate that the
compact object in NGC 7793 P13 is a neutron star accreting at super-Eddington
rates. While standard accretion models face difficulties accounting for the
pulsar X-ray luminosity, the presence of a multipolar magnetic field with $B$ ~
few $\times$ 10$^{13}$ G close to the base of the accretion column appears to
be in agreement with the properties of the system.
