  We report the discovery of a transient jet in the supersoft X-ray source RX
J0925-4758 . The jet was observed in the Halpha line during a single night in
June 1997 and had disappeared one day later. RX J0925-4758 is the third
supersoft source to exhibit collimated outflows. The peak jet velocity of 5,200
km/s, strongly argues in favour of a white dwarf in RX J0925-4758. Simple
modelling of the jet profile suggests half opening angles in the range of 17deg
to 41deg although the outflow may be narrower if part of the observed spread in
velocity is intrinsic to the jet. X-ray spectral modelling (Ebisawa et al.
1996; Hartmann & Heise 1997) indicates distances of the order of 10kpc or more
with the consequence that RX J0925-4758 may be the most optically luminous
supersoft source known. The overall observational picture points to a massive
white dwarf which may be close to the Chandrasekhar limit.
