By Jorge Otaola and Jean-Philippe Lefief BUENOS AIRES/PARIS (Reuters) - Eight French nationals including celebrity yachtswoman Florence Arthaud, Olympic swimmer Camille Muffat and boxer Alexis Vastine were among 10 killed when two helicopters collided in Argentina on Monday while filming a French reality TV show. The accident happened in the rugged western province of La Rioja near the Andes mountains, local authorities said, adding that the cause was not currently known.