Scientific abstract Please note that during visit to PDEC (May 29-31 1995), plan for T-o-O observations have been changed slightly - proposal now requests observations with PHT32 and CAM01 (once in microscanning and one in 'stare' mode) on three separate occasions. See more details in modified justification file. Note CAM03 no longer used. ORIGINAL PROPOSED ABSTRACT : We propose to use CAM01, CAM03 and PHT03 to observe Cygnus X-3 while in radio outburst. At such times this source ejects huge numbers of relativistic electrons into a double-lobed jet structure at approximately 0.3c. The high-frequency tail of the synchrotron emission from this source has been found to be higher than predicted during quiescence and has never been observed shortward of 1.3 mm during an outburst. Given the trigger for observation of 2 Jy at 3.6 cm we would expect highly variable fluxes of up to 1 Jy (certainly > 100 mJy) to be observed in the spectral region covered by ISO. These would be the first mid- to long-infrared observations of an evolved galactic jet source in outburst and would provide vital new data relating to beams, jets and ejection mechanisms. The 2 Jy at 3.6 cm trigger also sets off VLBI observations of Cyg X-3 which would automtically provide cm flux and spatial data to compare against the ISO data. Cyg X-3 is monitored in the radio daily. Observations of the high-frequency tail of the flare emission would effectively allow observations deeper down the jet towards its origin than would ever be possible from ground- based observations. This would shed new light on all stellar jets, particularly those from system containing evolved objects. Cyg X-3 is the brightest such source (when in outburst) to study such behaviour. Determination of degree of polarisation of synchrotron emission would provide info. on Faraday depol. processes associated with some local medium.