ROCTEC Futures Trading will join four other registered traders dealing in Hang Seng Index options when the new market is launched at the Hongkong Futures Exchange today. Joining Roctec - a subsidiary of the Roctec group which includes futures exchange chairman Leong Ka-chai among its founders - as registered traders in the option trading pit today are Cresvale Futures (Hongkong), Peregrine Futures (Hongkong), Swiss Bank Corp group and Wardley James Capel Futures (Hongkong). Credit Lyonnais Securities Futures will join the ranks later this month. It is the first time the futures exchange has used registered traders, or market-makers, who are traders obliged to make a market in a commodity. The opening day will have April, May, June, and September contract months available with seven strikes - 6,200, 6,300, 6,400, 6,500, 6,600, 6,700, and 6,800. Each contract month and strike has both calls and puts trading. The normal formula for adding strikes will be that if the index moves to within 200 points of the highest (or lowest) strike, then the next higher (or lower) strike will be added. These extra strikes will be added after the close of trading, although the exchange will reserve the right to add new strikes at any time in volatile conditions. The number of market-makers has aroused concern from analysts. The number of market-makers has aroused concern from analysts who are in favour of a greater number. But Ms Virginia Mumford, in charge of options and new product development at the exchange, said the efficiency of trading did not depend on the number of market-makers. ''It is a question of how good they are . . . the people we have are all very, very experienced,'' she said. The exchange might take on more registered traders if trading volume proved high enough to justify it. But Ms Mumford did not expect a great number of initial investors.