  We report the discovery of a very young high-mass X-ray binary (HMXB) system
associated with the supernova remnant (SNR) MCSNRJ0513-6724 in the Large
Magellanic Cloud (LMC), using XMM-Newton X-ray observations. The HMXB is
located at the geometrical centre of extended soft X-ray emission, which we
confirm as an SNR. The HMXB spectrum is consistent with an absorbed power law
with spectral index ~1.6 and a luminosity of 7x10^{33} ergs/s (0.2--12 keV).
Tentative X-ray pulsations are observed with a periodicity of 4.4 s and the
OGLE I-band light curve of the optical counterpart from more than 17.5 years
reveals a period of 2.2324\pm0.0003 d, which we interpret as the orbital period
of the binary system. The X-ray spectrum of the SNR is consistent with
non-equilibrium shock models as expected for young/less evolved SNRs. From the
derived ionisation time scale we estimate the age of the SNR to be <6 kyr. The
association of the HMXB with the SNR makes it the youngest HMXB, in the
earliest evolutionary stage known to date. A HMXB as young as this can switch
on as an accreting pulsar only when the spin period has reached a critical
value. Under this assumption, we obtain an upper limit to the magnetic field of
< 5x10^{11} G. This implies several interesting possibilities including
magnetic field burial, possibly by an episode of post-supernova hyper-critical
accretion. Since these fields are expected to diffuse out on a timescale of
10^{3}-10^{4} years, the discovery of a very young HMXB can provide us the
unique opportunity to observe the evolution of the observable magnetic field
for the first time in X-ray binaries.
