The Securities and Exchange Board of India (SEBI) has barred Copthall Mauritius Investment and Mansi Share & Stock Broking from accessing the securities market over alleged manipulative trading during the newly introduced Closing Auction Session (CAS) on the BSE Sensex. The action relates to trades executed on August 13, 2026, a Sensex weekly options expiry day. SEBI has also ordered the impounding of approximately ₹3.67 crore in alleged wrongful gains from the two entities.
The regulator’s ex-parte interim order came just six days after the alleged trades and represents its first major enforcement action involving the new CAS mechanism, which became operational on August 3. SEBI said its surveillance systems detected unusually sharp movements in the Sensex’s indicative equilibrium price during the auction and alleged that large orders placed by the two entities influenced the index in directions that could benefit their existing derivatives positions.
SEBI Impounds ₹3.67 Crore In Alleged Wrongful Gains
SEBI has calculated alleged wrongful gains of ₹2.96 crore for Copthall Mauritius Investment and ₹71.64 lakh for Mansi Share & Stock Broking.
The regulator has ordered the amounts to be impounded while its detailed investigation continues. Both entities have also been restrained from participating in the equity segment’s CAS until further orders.
SEBI Action At A Glance
| Parameter | Details |
|---|---|
| Regulator | SEBI |
| Entities | Copthall Mauritius Investment; Mansi Share & Stock Broking |
| Trading mechanism | Closing Auction Session (CAS) |
| Exchange | BSE |
| Index | Sensex |
| Date of alleged trades | August 13, 2026 |
| Copthall alleged gains | ₹2.96 crore |
| Mansi alleged gains | ₹71.64 lakh |
| Total amount impounded | ₹3.67 crore |
| Order type | Ex-parte interim order |
| Investigation | Ongoing |
ALLEGED WRONGFUL GAINSCopthall Mauritius ₹2.96 Cr | ██████████████████████████████Mansi Share & Stock ₹0.716 Cr | ███████TOTAL ₹3.67 Cr
SEBI’s directions are interim in nature, meaning the regulator has made a prima facie assessment while the broader investigation remains underway.
What Happened During The Sensex Closing Auction?
The case centres on the Closing Auction Session, a mechanism introduced to determine closing prices through an auction rather than relying solely on the earlier closing-price methodology.
SEBI’s surveillance systems identified three sharp movements in the Sensex’s indicative equilibrium price during the August 13 session. The movements occurred over periods ranging from just two seconds to 28 seconds.
Three Sharp Sensex Movements
| Event | Sensex Movement | Duration |
|---|---|---|
| First spike | 362.02 points | 2 seconds |
| Second spike | 132.67 points | 12 seconds |
| Third spike | 405.08 points | 28 seconds |
SENSEX CAS PRICE MOVEMENTSSpike 1 +362.02 pts | ██████████████████Spike 2 +132.67 pts | ███████Spike 3 +405.08 pts | ████████████████████
The first movement occurred between approximately 3:20:41 pm and 3:20:43 pm, while the second occurred between 3:24:08 pm and 3:24:20 pm. The third movement took place between 3:25:49 pm and 3:26:17 pm.
Copthall Allegedly Placed Aggressive Buy Orders
According to SEBI’s interim order, Copthall Mauritius placed large buy orders across Sensex constituent stocks at prices around 3% above the reference price during the CAS.
The regulator said Copthall accounted for 99.91% of total buy-order value during the first sharp movement, followed by 96.09% during the second and 85.21% during the third.
Copthall’s Reported Share Of Buy-Order Value
| Period | Copthall’s Share |
|---|---|
| First spike | 99.91% |
| Second spike | 96.09% |
| Third spike | 85.21% |
COPTHALL BUY-ORDER CONCENTRATIONFirst spike 99.91% | ██████████████████████████████████████████████████Second spike 96.09% | ████████████████████████████████████████████████Third spike 85.21% | ██████████████████████████████████████████
SEBI said Copthall subsequently cancelled a substantial portion of these orders after the price movements occurred. The regulator also examined Copthall’s derivatives positions, which included long call and short put positions in Sensex options that could benefit from an upward movement in the index.
Mansi Allegedly Used Large Sell Orders
SEBI’s order also identified large sell orders placed by Mansi Share & Stock Broking across Sensex constituents.
The regulator said Mansi placed sell orders aggregating approximately 12.65 lakh shares across eight Sensex constituent stocks at prices below the reference price. The orders were subsequently cancelled within a few seconds.
SEBI also examined Mansi’s derivatives positions and said its put-option positions could benefit from a decline in the Sensex.
Mansi Trading Activity
| Parameter | Reported Figure |
|---|---|
| Sensex constituents involved | 8 |
| Sell orders | 12.65 lakh shares |
| Pricing | Below reference price |
| Subsequent action | Orders cancelled |
| Derivatives exposure | Sensex put options |
The regulator’s preliminary assessment is that the two entities appeared to influence the index in directions favourable to their respective derivatives positions. However, the order does not allege that the two entities acted in concert.
How The Alleged Strategy Worked
The alleged trading activity involved the relationship between the cash-market prices of Sensex constituents and positions in Sensex derivatives.
The basic mechanism identified by SEBI can be simplified as follows:
LARGE ORDERS IN SENSEX STOCKS ↓CHANGE IN INDICATIVE EQUILIBRIUM PRICE ↓SENSEX CLOSING PRICE MOVES ↓IMPACT ON SENSEX OPTIONS ↓POTENTIAL DERIVATIVES GAINS
SEBI’s concern is that the CAS gives market participants an opportunity to influence the closing price if they can place sufficiently large orders in the underlying stocks.
The regulator said such conduct could undermine fair price discovery and affect participants trading Sensex derivatives.
What Is The Closing Auction Session?
The Closing Auction Session, or CAS, was introduced by SEBI as a new mechanism for determining closing prices of eligible stocks.
The framework became effective on August 3, 2026.
Under the mechanism, normal trading in covered stocks ends at 3:15 pm, followed by a reference-price calculation period. The auction then takes place before a single equilibrium price is determined.
CAS Timeline
| Time | Stage |
|---|---|
| 3:15 pm | Normal trading ends |
| 3:15-3:20 pm | Reference price calculation |
| 3:20 pm | CAS begins |
| 3:28-3:30 pm | Random closure window |
| 3:35 pm | Closing price determined |
The exact operational framework is intended to use an auction-based process to improve price discovery and reduce opportunities for manipulation.
Why CAS Matters For Derivatives Traders
The closing price of the Sensex is particularly important on derivatives expiry days because it can determine settlement values for index options.
That means even a short-lived movement in the underlying stocks can potentially have financial consequences for traders holding derivatives positions.
CAS And Derivatives Connection
Sensex Constituent Stocks
↓
CAS Orders
↓
Indicative Equilibrium Price
↓
Sensex Closing Level
↓
Options Settlement
↓
Trader Gains / Losses
SEBI said manipulative practices during the auction could therefore have wider consequences for participants in the futures and options market, including retail investors.
Sensex Closed At Around 78,080
According to the SEBI order, the Sensex had a reference price of 77,829.60 at 3:15 pm on August 13.
The CAS subsequently produced a closing price of 78,079.96, which was rounded to 78,080 for the regulator’s calculations.
Sensex Closing Price
| Metric | Level |
|---|---|
| Reference price at 3:15 pm | 77,829.60 |
| CAS closing price | 78,079.96 |
| Rounded closing level | 78,080 |
| Difference | ~250.36 points |
SENSEX CAS LEVELReference 77,829.60 | ███████████████████████████████████████CAS Close 78,079.96 | █████████████████████████████████████████Difference 250.36 | █
SEBI’s order also compared the Sensex move with the Nifty and calculated that the Sensex eventually closed around 240 points higher than the level it believed would have been indicated by comparable Nifty movement.
SEBI Bars Both Entities From CAS Participation
The regulator has not limited its action to impounding the alleged gains.
Both entities have been prohibited from accessing the securities markets and from participating directly or indirectly in the equity segment’s CAS until further orders. They are also barred from placing, modifying or cancelling orders during the auction.
For Mansi Share & Stock Broking, the restriction applies specifically to its proprietary trading account.
Regulatory Restrictions
| Restriction | Copthall | Mansi |
|---|---|---|
| Access to securities market | Barred | Barred |
| Equity CAS participation | Barred | Barred |
| Place CAS orders | Barred | Barred |
| Modify CAS orders | Barred | Barred |
| Cancel CAS orders | Barred | Barred |
| Scope for Mansi | — | Proprietary account |
SEBI has also directed that bank accounts associated with the entities be restricted from debits without the regulator’s permission.
Outstanding Sensex Options Positions Triggered Urgency
SEBI said the entities had outstanding positions in the weekly Sensex options expiring on August 20, which contributed to the need for immediate interim action.
The regulator said the restrictions were necessary to prevent any further possible misuse of the CAS mechanism while the investigation continues.
This is particularly important because the alleged August 13 activity occurred shortly before another weekly options expiry.
SEBI Chairman Warned Of Strict Action
The enforcement action came just hours after SEBI Chairman Tuhin Kanta Pandey warned market participants against manipulating the new CAS mechanism.
Pandey said SEBI would take strict and immediate action if participants attempted to manipulate the auction, adding that the new mechanism gives the regulator greater ability to detect manipulation compared with the earlier VWAP-based system.
SEBI'S MESSAGENew CAS ↓Greater Surveillance ↓Manipulative Orders Detected ↓Swift Regulatory Action ↓Market Access Restrictions
The timing of the warning and interim order underscores SEBI’s intention to establish strong enforcement around the new auction system from its early stages.
SEBI Says CAS Should Improve Price Discovery
The regulator has said CAS is designed to make closing-price discovery more transparent and efficient.
The auction mechanism determines a single equilibrium price based on executable quantities rather than relying solely on continuous trading immediately before the market close.
SEBI therefore views manipulation of the CAS as particularly serious because it can undermine the very price-discovery mechanism the system was designed to improve.
Intended CAS Benefits
| Objective | Expected Benefit |
|---|---|
| Auction-based pricing | Better price discovery |
| Reference price | Establishes starting point |
| Equilibrium price | Single closing price |
| Surveillance | Easier detection of unusual activity |
| Transparency | More structured closing process |
| Market integrity | Reduced scope for unfair practices |
The Case Is Still Under Investigation
It is important to note that SEBI’s current action is an ex-parte interim order based on a prima facie assessment.
The regulator has not completed its detailed investigation into the matter.
The restrictions and impounding of alleged gains are therefore interim measures while SEBI continues examining the trading activity.
CURRENT STATUSAlleged Trades ↓SEBI Surveillance ↓Prima Facie Findings ↓Interim Order ↓Market Access Restrictions ↓₹3.67 Cr Impounded ↓Detailed Investigation
The final findings could depend on further examination of order logs, trading positions, intent and other evidence.
Why The Action Matters For India’s Markets
The case is significant beyond the two entities involved because CAS is a new market mechanism.
An early manipulation case could influence how brokers, proprietary trading firms, foreign portfolio investors and other sophisticated market participants approach the auction.
It also demonstrates that SEBI is closely monitoring the interaction between cash-market orders and derivatives positions.
Key Implications
| Area | Potential Impact |
|---|---|
| CAS | Increased scrutiny |
| Brokers | Greater compliance requirements |
| Proprietary traders | Higher surveillance risk |
| F&O participants | Greater focus on settlement-price manipulation |
| Retail investors | Protection against distorted closing prices |
| SEBI | Stronger enforcement precedent |
The regulator’s swift action could act as a deterrent to other market participants considering similar strategies.
The Bigger Picture
SEBI’s action against Copthall Mauritius Investment and Mansi Share & Stock Broking marks an important early test for India’s new Closing Auction Session. The regulator alleges that large and aggressive orders in Sensex constituent stocks were used to influence the indicative equilibrium price during the August 13 expiry-day auction, potentially benefiting the entities’ derivatives positions.
The ₹3.67 crore impounding order is relatively small compared with the size of India’s overall derivatives market, but the regulatory significance is much larger. SEBI has made clear that it intends to closely monitor CAS and act quickly against activity that could interfere with fair price discovery. The fact that the order came within six days of the alleged trades also highlights the regulator’s focus on rapid intervention.
Looking Ahead
SEBI’s detailed investigation will determine whether the preliminary findings are ultimately sustained and whether additional regulatory or enforcement action follows. For market participants, the case is an early warning that orders placed during CAS will face close surveillance, particularly when large cash-market trades coincide with significant derivatives positions around expiry.
The episode could also shape the evolution of the CAS mechanism itself. If the system continues to provide SEBI with better visibility into unusual order activity, the regulator may use the case to refine surveillance and compliance practices. For traders and investors, the broader takeaway is that attempts to influence closing prices can have consequences well beyond individual stocks, particularly when those prices are used to settle index derivatives
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