MUMBAI, Aug 4: Stock market benchmark indices Sensex and Nifty closed lower on Tuesday, ending their four-day rally, following the introduction of a new auction mechanism for shares having futures and options (F&O) contracts. The Closing Auction Session (CAS) in the equity cash segment became operational on Monday, introducing a new auction-based mechanism for determining the closing prices of eligible stocks in a move aimed at making the price discovery process more transparent and robust. Benchmark equity indices Sensex and Nifty on Monday ended with an unusual aberration after stock exchanges introduced the new auction mechanism. “The introduction of the Closing Auction Session (CAS) for F&O stocks has added a new layer of volatility to benchmark indices, with Tuesday’s trade reflecting an early adjustment to the revised market structure. Extending the winning momentum to the fourth day on Monday, the Sensex jumped 544.39 points, or 0.70 per cent, to settle at 78,639.03.