  A near-infrared jet emanating from the galactic superluminal source GRS
1915+105, discovered at $\lambda$ = 2.2 micrometer by Sams et al. (1996) on
18-21 July 1995, was not seen later anymore, neither in the Mirabel et al.
(1996) images, taken on 4 August 1995 and that we analyze here, nor by
Eikenberry & Fazio (1997) on 16-17 October 1995. Therefore, the radiative
lifetime of the electrons which have created this emission is $\tau$ < 17 days.
The detection, and the later non-detection, of this infrared jet is consistent
with a synchrotron emission, provided that the magnetic field in the ejected
clouds is B > 160 mG, and that the Lorentz factor of the internal motions of
the electrons in the jet is $\gamma < 10^{4}$. The lower limit for the magnetic
field suggests a magneto-hydrodynamic mechanism for the acceleration and
collimation of the ejecta.
