Introduction of Closing Auction Session (CAS) in the Equity Cash Segment

Folks, SEBI is changing the way closing prices are discovered for F&O stocks from 3rd August based on this circular.

Until now, our markets had a ‘Continuous Trading Session (CTS)’, and the closing price of stocks and indices was based on the final 30 mins VWAP. With the new ‘Closing Auction Session (CAS)’, all buy and sell orders will be pooled together and matched at one equilibrium price between 3:15 and 3:35 PM.

It’s a big change, and it matters because it’ll make it harder for sharks to influence the closing price with a few big trades in the last few minutes, because price discovery is based on aggregated demand and supply, not just a mathematical VWAP formula.

The equity segment moves to CAS from 3:15–3:35 PM, while F&O continues till 3:40 PM. Intraday trades will also need to be squared off before these timings. Long-term investors don’t need to change anything, but active traders will need to adapt.

While all this may seem inconvenient and complicated, it’s a move toward established global practices. So, overall, I think this is a sensible step by SEBI toward better price discovery for everyone, from retail traders to passive/ETFs and active funds. For more information, read our Risk Management Policy Update. If you have any doubts or further questions regarding this, do reach out in the comments. Will try to respond ASAP.

4 Likes

At 3:10 PM

  • Nifty ≈ 24,573.55
  • 24550 CE was around ₹70–80

This is perfectly normal because the option was only about 24 points ITM.

Intrinsic Value ≈ ₹24

Premium ≈ ₹70

Time Value ≈ ₹46

No issue.

At 3:25 PM (CAS)

  • Nifty has jumped to 24,774.30
  • This is the Indicative Equilibrium Price (IEP) being reflected on the index chart.

Now, mathematically,

Intrinsic Value of 24550 CE should become

24774 - 24550

=

224

So even if Time Value becomes almost zero,

the option should theoretically be worth at least ₹224.

Yet the option chain is still showing around ₹78.5.

That is exactly wrong if a person looks the option chain and understanding the option price considering the Intrinsic value and Time value, IV etc.
How SEBI will help traders to overcome this kind of confusion?

The new closing price of Nifty after CAS also gives wrong daily candle closure. See the attached chart. The market closed at 24573 at 3:15, the EOD chart is Doji, where as, due to CAS closing, the EOD candle is a huge Bullish candle

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Yeah, I know. CAS can lead to a divergence in closing price relative to the continuous trading price. This happens in mature markets too and mostly because of large volumes in a small time window. Can’t really make an uninformed judgement as to why it exactly happened for two continuous days. But, I guess it could be because of short covering by brokerages and prop firms perhaps because they hadn’t adapted. Anyways, we’ll know in a few more days as traders and investors adjust to this new method.

Too early to comment on this. Let’s see if this will continue to happen in the coming sessions.