Sebi flags manipulative trades during CAS on Sensex expiry day, fines two entities

Sebi has passed an ex-parte interim order against Copthall Mauritius Investment Limited and Mansi Share and Stock Broking Private Limited for alleged manipulative trades during the closing auction session on BSE’s Sensex weekly expiry day.

The regulator has impounded Rs 3.67 crore of alleged wrongful gains and restrained both entities from accessing the securities market until the amount is deposited in fixed deposits marked with lien in favour of Sebi. In the case of Mansi, the restraint applies to its proprietary trading account.

The case relates to trading during the Closing Auction Session, or CAS, on August 13, 2026, a Sensex weekly options expiry day. Sebi said the settlement price of Sensex options was based on the closing price of the index determined during CAS.


CAS was introduced by Sebi through a circular dated January 16, 2026 and became effective from August 3, 2026. Under the framework, normal trading in covered cash market stocks stops at 3:15 pm. A reference price is then calculated between 3:15 pm and 3:20 pm. The auction runs from 3:20 pm to 3:30 pm, with random closure between 3:28 pm and 3:30 pm. A single equilibrium price is discovered by 3:35 pm.

Sebi said surveillance teams noticed abnormal spikes in the indicative equilibrium price of Sensex during the CAS on August 13. The reference price of Sensex was 77,829.60 at 3:15 pm, while the CAS-discovered closing price was 78,079.96, rounded to 78,080 for calculations.

The order said there were three sharp spikes in Sensex during the auction. The first spike came between 3:20:41 pm and 3:20:43 pm, when Sensex moved from 77,661.40 to 78,023.42, a jump of 362.02 points in two seconds. The second spike came between 3:24:08 pm and 3:24:20 pm, when the index rose 132.67 points. The third spike came between 3:25:49 pm and 3:26:17 pm, when Sensex moved up 405.08 points in 28 seconds.

Sebi said Copthall was the dominant buyer during the auction. It accounted for 86.6% of gross buy value in Sensex constituents during CAS. The order also said Copthall cancelled the highest number of buy orders, cancelling 10.38 lakh shares out of 31.66 lakh shares ordered, or 32.79%.

During the first spike, Copthall accounted for Rs 66.58 crore of the total Rs 66.64 crore buy order value, or 99.91%. Its 32 limit buy orders were placed across all Sensex stocks at nearly 3% above the reference price, while other market participants placed orders below 2%. Sebi said this showed an aggressive and index-wide price impact.

The same pattern repeated in the second spike, where Copthall accounted for Rs 126.59 crore of Rs 131.74 crore buy order value, or 96.09%. In the third spike, Copthall accounted for Rs 98.12 crore, or 85.21% of the total buy order value.

Sebi said Copthall had expiry-day Sensex option positions that would benefit from a higher Sensex closing value. It held net buy call positions and net sell put positions at the 77,500, 78,000 and 78,500 strike prices.

The regulator said the cash market activity during CAS could not be viewed separately from its derivatives positions.

Mansi, on the other hand, allegedly placed large sell orders at lower prices and then cancelled them. Sebi said Mansi placed sell orders worth Rs 145.65 crore between 3:21:03 pm and 3:26:00 pm across Sensex stocks and cancelled Rs 143.43 crore of those orders between 3:26:02 pm and 3:26:05 pm.

The order said Mansi cancelled 12.65 lakh out of 12.77 lakh sell orders, a cancellation rate of 99.06%. Sebi said the orders appeared to suppress the Sensex indicative price for around five minutes. Once those orders were cancelled, Sensex IEP rose sharply by 232.96 points.

Mansi had open put option positions on the expiry day. Sebi said any downward movement in Sensex would have helped those positions. The regulator said Mansi exited its put option positions during the period when its sell orders were allegedly suppressing the index price.

Sebi said it was not alleging at this stage that Copthall and Mansi acted in concert. It said the two entities adopted opposite but aggressive price-impacting strategies during the same CAS session. Copthall’s orders pushed the index up while Mansi’s orders pushed it down temporarily until cancellation.

The regulator calculated wrongful gains of Rs 2.96 crore for Copthall and Rs 71.64 lakh for Mansi, taking the total alleged wrongful gain to Rs 3.67 crore.

Sebi said manipulation in CAS has serious market implications because the auction-discovered price is used for options settlement, mutual fund net asset value calculation and other market functions. It said such conduct, if allowed to continue, could damage fair price discovery and harm investors who trade in derivatives or invest through mutual funds.