The foreign institutional investors (FIIs) were net buyers to the tune of Rs 513.67 crore in the futures & option segment on Wednesday.According to data released on the NSE website, FIIs were net buyers of index futures to the tune of Rs 474.89 crore while they sold index options worth Rs 26.62 crore. They were net buyers of stock futures to the tune of Rs 46.36 crore and they bought stock options worth Rs 19.03 crore.