  We have discovered a strong lensing fossil group (J0454) at z=0.26, projected
near the well-studied cluster MS0451-0305. Using multicolour Subaru/Suprime-Cam
and CFHT/Megaprime imaging together with Keck spectroscopy we identify member
galaxies. A VLT/FORS2 spectrum was taken to determine the redshifts of the
brightest elliptical and the lensed arc. Using HST/ACS images we determine the
group's weak lensing signal and model the strong lens system. This is the first
time that a fossil group is analysed with lensing methods. The X-ray luminosity
and temperature are derived from XMM-Newton data.
  We find two filaments extending over 4 Mpc, and within the virial radius we
identify 31 members spectroscopically and 33 via the red sequence with i<22
mag. They segregate into spirals and a dynamically cooler central concentration
of ellipticals with a velocity dispersion of 480 km/s. Weak lensing and cluster
richness relations yield r200 = 830 kpc and M200 = 0.85 x 10^14 Msun. The
brightest group galaxy (BGG) is inconsistent with the dynamic centre of J0454.
It strongly lenses a galaxy at z~2.1, and we model the lens with a
pseudo-isothermal elliptical mass distribution. A large external shear requires
that the BGG must be offset from J0454's dark halo centre by at least 90-130
kpc. The X-ray halo is offset by 24 kpc from the BGG, shows no signs of a
cooling flow and is well described by a beta-model. With L = 1.4 x 10^43 erg/s
J0454 falls well onto standard X-ray cluster scaling relations, but appears
cooler (1.1 keV) than expected (2 keV). We conclude that J0454 consists of two
systems, a sparse cluster and an infalling fossil group, the latter seeding the
BCG. An alternative explanation is that galaxies in a filament, projected along
the line of sight, stream towards the denser fossil group.
