Nifty and Sensex diverge after NSE introduces Closing Auction Session timings

Mumbai: Investors were surprised on Tuesday when the Nifty 50 and BSE Sensex moved differently. Around 9:42 am, Nifty fell 100 points to 24,460, while Sensex gained 166.15 points to 78,805.18.By 2 pm, Sensex was down 400 points, or 0.5 percent, while Nifty had fallen 330 points, or 1.5 percent. The difference raised concerns about a possible technical or calculation error.NSE Explains DifferenceThe National Stock Exchange (NSE) clarified that there was no market anomaly. It linked the unusual movement to the new Closing Auction Session (CAS), introduced on August 3, 2026.Nifty and Sensex contain different companies and assign them different weights. Therefore, changes in share prices can affect the two indices differently. Separate order books for normal trading and CAS can make this gap more visible.How CAS Works?Earlier, a stock’s closing price was based on the volume-weighted average price during the final 30 minutes of trading. Under CAS, the closing price of eligible derivative stocks is decided through an auction.Buy and sell orders are first collected. The system then identifies the price at which the largest number of trades can be completed. This becomes the equilibrium price and is used to determine the closing price.NSE CEO Ashish Chauhan Urges Startups, SMEs To Use IPOs For Growth Between 3:15 pm and 3:30 pm, orders are not matched continuously. Investors may place, change or cancel orders before the system calculates the equilibrium price. Therefore, the index displayed during this period may remain unchanged briefly and should not immediately be treated as final.Options Traders WarnedCAS runs from 3:15 pm to 3:35 pm, while derivatives trading continues until 3:40 pm. This timing difference can create a temporary gap between spot and futures prices.Such changes are important on expiry days. Even a two-point adjustment near an option’s strike price can sharply affect its premium. Traders caught on the wrong side may find it difficult to square off positions at the expected price.NSE said the first CAS attracted 515 trading members and orders from 56,773 unique PANs. Retail investors need not panic over short-lived differences between Nifty and Sensex. However, options traders should remain careful during the closing minutes.

Nifty 50 jumped nearly 200 points in just two minutes on Monday, as traders adjusted to India's new closing auction system for large stocks. At 3.28 pm, Nifty was trading at 24,573, which then surged to 24,774 to close nearly 1.6% higher at 3:30 pm. The sharp move came after exchanges introduced the Closing Auction Session (CAS) from August 3. The new system changes how the closing price is calculated for stocks that are part of the futures and options list.The move appeared to surprise some intraday traders, especially those used to the old 3:30 pm closing pattern. Under the new structure, different parts of the market now close at different times. For F&O-linked stocks, continuous trading stops at 3:15 pm. These stocks then move into a closing auction process. Other stocks continue trading until 3:30 pm. Index and stock F&O contracts remain open until 3:40 pm.What changed from August 3The biggest change is that F&O stocks no longer use the old final 30-minute VWAP method to decide their official closing price. Earlier, the closing price was calculated using the volume-weighted average price of trades done in the last 30 minutes of trading. VWAP means the average price weighted by traded quantity.This was used because the last traded price can be misleading. A stock may trade around Rs 1,000 for most of the day, but one small trade at Rs 980 near the close can make the last traded price look weak. VWAP reduces that distortion.From August 3, the regulator has introduced a closing auction for stocks that have derivatives contracts. In this system, buy and sell orders are pooled and matched at a single price. That price becomes the official closing price.The idea is to make the closing price more fairer, especially for large orders and passive funds that need to trade near the close.Also Read: Explained: What is CAS and what do new stock market timings mean for BSE, NSE traders?How the auction worksFor stocks under CAS, exchanges first calculate a reference price between 3 pm and 3:15 pm. This is based on the VWAP of trades during that period. At 3:15 pm, continuous trading stops. There is then a five-minute transition period until 3:20 pm. No fresh orders can be entered during this window.From 3:20 pm to 3:25 pm, investors can place market and limit orders. Exchanges show details such as indicative price, buy quantity, sell quantity and order imbalance. From 3:25 pm to 3:30 pm, only limit orders can be placed. Market orders already placed cannot be changed or cancelled during this period.The order entry window closes randomly between 3:28 pm and 3:30 pm This is meant to stop a last-minute rush of orders. From 3:30 pm to 3:35 pm, exchanges match the orders. The price at which the maximum quantity can be traded becomes the equilibrium price. That becomes the stock’s official closing price.If two prices can execute the same quantity, the price with the lowest unmatched quantity is selected. If there is still a tie, the price closest to the reference price is chosen. If no price is discovered, the reference price is used.Why Nifty moved so fastThe Nifty may have moved sharply because the closing prices of index stocks directly affect index calculation. The Nifty and Sensex are based on the market value of their constituent stocks. When the auction discovers closing prices for large index stocks, the index can move quickly even though regular continuous trading has already stopped for those stocks.The 200-point jump in two minutes suggests that orders placed during the auction changed the final prices of some large stocks enough to move the index. Traders may also have been caught off guard by the new timings.Intraday positions using MIS product type now have earlier auto square-off timings. For equity stocks under CAS, auto square-off is at 3:10 pm For equity stocks not under CAS, it is 3:25 pm For index and stock F&O contracts, it is 3:26 pmThat means some traders who were used to managing positions closer to 3:30 pm now have to adjust earlier.(Disclaimer: Recommendations, suggestions, views and opinions given by the experts are their own. These do not represent the views of Economic Times)