Asia's oldest exchange, BSE, traded far fewer derivatives contracts in the first week following the implementation of the Closing Auction Session (CAS). However, the contracts generated significantly higher premiums, helping the exchange sharply improve its premium turn over and gain market share, according to Nuvama Institutional Equities.

Nuvama said the first week of CAS implementation appears to have curbed last-minute swings, fuelling flows into deep out-of-the-money options. This resulted in a sharp decline in the average number of contracts traded, even as premiums rebounded.

4% decline for the industry. 8% week-on-week to Rs 2,605.

This compares with Rs 1,688 in July 2026 and Rs 1,902 in Q1 FY27. 5% growth recorded by the industry.

The figure was also above Nuvama’s remaining FY27E ADPTV estimate of Rs 21,800 crore. 1%, an increase of 362 basis points week-on-week.

2%. 6 basis points for the industry.

What is CAS? Market regulator Securities and Exchange Board of India (SEBI) and the National Stock Exchange (NSE) introduced the Closing Auction Session (CAS) for eligible Futures & Options (F&O) stocks which started on August 3.

The new framework changes how closing prices are determined for these stocks and extends equity derivatives trading by 10 minutes. For investors, the biggest change is this: the closing price of eligible F&O stocks will no longer be calculated using trades executed during the final 30 minutes of the trading session.

Instead, it will be discovered through a dedicated auction held after continuous trading ends. Simply put, CAS is a 20-minute auction that begins after continuous trading ends.

Instead of executing every trade instantly as it happens during the trading day, all buy and sell orders are pooled together and matched at one common price. That single price becomes the stock's official closing price.

m. The process begins before the auction starts.

m. , exchanges will calculate the volume weighted average price (VWAP) of trades in every eligible stock.

, no fresh orders can be placed as the market transitions into the auction. m.

, investors can place both market and limit orders. m.