About the order types, right now we use a very simple execution strategy (that is also included in our automated trader). Basically, the limit orders (buy and sell at the same time) sit until they are executed or a 5-minute timeout is passed by. After this time a market order is placed. Given the liquidity of the NASDAQ 100 market 70 % of the orders get executed immediately. Otherwise, the slippage tends to average out to about 0.02 % per day. But again, one could conceive an execution strategy that is more sophisticated and that includes being able to short at the right time.