India's stock market is overhauling how official closing prices are determined for stocks in the futures and options (F&O) segment, moving from a volume-weighted average price (VWAP) system to a Closing Auction Session (CAS) starting August 3. The new 20-minute call auction, held between 3:15 PM and 3:35 PM, aims to aggregate genuine demand and supply to arrive at a single equilibrium closing price, reducing the risk of end-of-day price manipulation and improving the accuracy of benchmark prices for indices, derivative settlements, and mutual fund NAVs. The Securities and Exchange Board of India (SEBI) and exchanges believe this structurally stronger design will make it harder to influence closing prices with last-minute large orders.
The change is expected to significantly impact cash-futures arbitrage strategies. Traders who previously capitalized on small price differences between the cash and futures markets will face challenges because the final cash market execution price will only be known after the auction concludes. This uncertainty makes hedging futures positions more difficult and, combined with Securities Transaction Tax (STT) costs, could make such arbitrage trades less attractive. Hedge fund manager Mayank Bansal noted that while arbitrage funds might experience minor volatility in their end-of-day Net Asset Values (NAVs) due to differing closing practices for cash and future legs, long-term returns are expected to remain unchanged.
Brokers are anticipating a modest hit to their revenues, potentially seeing a 1-5% reduction, as the new framework could curb high-frequency and liquidity-providing trades that typically occur near the close. Continuous trading for Category I stocks will end at 3:15 PM, with futures and options trading continuing until 3:40 PM, while cash market trading in these stocks will be suspended during the auction. Market participants also expect a decline in negotiated institutional trades initially, and many intraday traders may opt to square off positions before 3:15 PM due to uncertainty about the auction price. However, experts like Zerodha Co-founder Nithin Kamath suggest the CAS will make it harder for large orders to disproportionately influence closing prices, thereby benefiting passive and index funds by ensuring a more robust and tamper-resistant reference price.
