4 Conclusions

Our EVN observations at 5 GHz revealed that LS I +61$^{\circ }$303 has a jet-like elongation to the southeast on scales of tens of milliarcseconds. This is interpreted as an approaching, Doppler boosted, jet of a symmetric pair. The angle between the jet and the line of sight at the epoch of our observation was close to zero. However, data at other epochs suggest the existence of precession of the jet, and the accretion disk, of at least $5^{\circ}$. This precession would change the angle of the ejecta and consequently would vary both the apparent expansion velocity and the intensity of the peak of the radio outbursts. New VLBI observations at several epochs are necessary to correlate morphology, position angle, and expansion velocity, with the 4 yr modulation of the peak of the radio outbursts (Gregory et al. 1999) and of the H$\alpha$ emission line (Zamanov et al. 1999).

We derived a lower limit of 0.4 c for the intrinsic velocity of the radio jet. This value is well within the range 0.1 c to 0.9 c found for microquasars like SS 433, Cygnus X-3, GRS 1915+105 and GRO J1655-40 (Mirabel & Rodríguez 1999).

Finally, in some microquasars, emission almost orthogonal to the jet, i.e. along the orbital plane, exists. This emission has clearly been observed in SS 433 (Paragi et al. 1999) and it is barely visible in Cygnus X-1 (Stirling et al. 2000). As we showed in this paper, LS I +61$^{\circ }$303 could be a third case of this kind of source.

Acknowledgements

We thank Karl Menten, Andrew Lobanov, Alok Patnaik, Eduardo Ros and Giovanna Pugliese for useful discussions and comments. We acknowledge detailed and very useful comments from L. Lara, the referee of this paper. During this work, M.R. has been supported by two fellowships from CIRIT (Generalitat de Catalunya, Ref. 1998 BEAI 200293 and 1999 FI 00199). J. M. P. and M. P. acknowledge support from DGICYT grant PB97-0903 (Spain).