  PSR B1259-63 is a 48 ms pulsar in a highly eccentric 3.4 year orbit around
the young massive star LS 2883. During the periastron passage the system
displays transient non-thermal unpulsed emission from radio to very high energy
gamma rays. It is one of the three galactic binary systems clearly detected at
TeV energies, together with LS 5039 and LS I +61 303. We observed PSR B1259-63
after the 2007 periastron passage with the Australian Long Baseline Array at
2.3 GHz to trace the milliarcsecond (mas) structure of the source at three
different epochs. We have discovered extended and variable radio structure. The
peak of the radio emission is detected outside the binary system near
periastron, at projected distances of 10-20 mas (25-45 AU assuming a distance
of 2.3 kpc). The total extent of the emission is ~50 mas (~120 AU). This is the
first observational evidence that non-accreting pulsars orbiting massive stars
can produce variable extended radio emission at AU scales. Similar structures
are also seen in LS 5039 and LS I +61 303, in which the nature of the compact
object is unknown. The discovery presented here for the young non-accreting
pulsar PSR B1259-63 reinforces the link with these two sources and supports the
presence of pulsars in these systems as well. A simple kinematical model
considering only a spherical stellar wind can approximately trace the extended
structures if the binary system orbit has a longitude of the ascending node of
omega ~ -40 deg and a magnetization parameter of sigma~0.005.
