New Delhi, Aug. 20 -- The Securities and Exchange Board of India's (Sebi) crackdown on a foreign portfolio investor and a domestic brokerage for manipulating the closing auction session (CAS) is a warning to traders against any mischief during the upcoming monthly expiry of Nifty and Sensex derivatives and beyond, say market experts.

Besides weekly index options expiry, NSE and BSE futures and options (F&O) contracts normally expire on the last Tuesday and Thursday of a month, respectively. The settlement happens a day earlier if either of the days coincides with a holiday.

On expiry days, the prices of cash shares and their derivative counterparts converge as the premium (a function of interest rates, dividends, and time to expiry) bet...