The Foreign Institutional Investors (FIIs) were net buyers to the tune of Rs 663.54 crore in the futures & options segment on Thursday.According to data released by the NSE, FIIs were net buyers of index futures to the tune of Rs 149.06 crore and bought index options worth Rs 286.05 crore. They were net buyers of stock futures to the tune of Rs 234.18 crore and sold stock options worth Rs 5.75 crore.