Results of the final analysis of COS-B X-ray observations of Cygnus X-3 (more than 220 days of observations from 1975 to 1982) are presented. Variations of the 4.8h period have been investigated and a new ephemeris including previous and later (Exosat) results, is established. Both long and short-term changes in the period are apparent. The long term variation is found to be consistent with orbital changes expected from mass transfer and mass loss in a standard low-mass binary system with an accreting neutron star. Short term period fluctuations (of the order of 4 10−3 d.) are definitively present and not fully explained. A critical discussion of the recent claimed high energy gamma (E > 1012eV) detections of the source is also presented. It is argued that, if confirmed, these results are inconsistent with the typical X-ray picture of Cygnus X-3. In particular, the eclipse duration required in the Vestrand and Eichler (1982) imply a totally unphysical companion for a 4.8h orbit.