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In 2010/3, the Large Area Telescope on board Fermi revealed a transientgamma-ray source, positionally coincident with the optical nova in thesymbiotic binary, V407Cyg. This event marked the first discovery of gamma-rayemission from a nova. We aimed to obtain resolved radio imaging of the materialinvolved in the nova event; to determine the ejecta geometry and advancevelocity directly in the image plane; to constrain the physical conditions ofthe system. We observed the source with the EVN and the VLBA over 16 epochs,between 20 days and 6 months after the optical discovery. The source isinitially very dim but it later shows a substantial increase in brightness anda resolved shell-like structure 40 to 90 days after the optical event. Theshell has a projected elliptical shape and is asymmetric in brightness andspectral index, being brighter and characterised by a rising spectrum at theS-E edge. We determine a projected velocity of ~3500 km/s in the initial phase,and ~2100 km/s between day 20 and 91. We also found an emitting feature about350 mas (940 AU) to the N-W, advancing at a projected velocity of ~700 km/salong the polar axis of the binary. The total flux density in the VLBI imagesis significantly lower than that previously reported at similar epochs and overmuch wider angular scales with the VLA. Optical spectra demonstrated that in2010 we were viewing V407Cyg along the equatorial plane and from behind theMira. Our radio observations image the bipolar flow of the ejecta perpendicularto the orbital plane, where deceleration is much lower than through theequatorial plane probed by the truncated profile of optical emission lines. Theseparated polar knot at 350 mas and the bipolar flow strictly resemble thesimilar arrangement seen in Hen 2-104. The observed ~700 km/s expansionconstrains the launch-date of the polar knot around 2004. [Abridged]

Very Long Baseline Interferometry imaging of the advancing ejecta in the first gamma-ray nova V407 Cyg