Keystone Communications Corp. announced it has leased three additional C-band transponders aboard the Galaxy 4 spacecraft to expand its occasional-use C-band transponder capacity by 50 percent. The new transponders will support syndication, sports and other broadcast applications, Keystone said. The com pany already leases other transponders aboard Galaxy 4, as well as transponders aboard Galaxy 6 and Telstar 401. (Keystone, 310/845-3718.)