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Shares of Delta Air Lines (DAL - Get Report)  were down 0.8% Monday after the company was downgraded by analysts at Morgan Stanley who believe the carrier has reached mature valuation. 

The firm also lowered the price target on the stock to $61 from $62. The stock was trading at $54.44 per share Monday. 

"As we look at our refreshed 2020 estimates, which are moderately below consensus and reflect pilot step-ups, higher oil, elevated supply growth (3-4% 2019/2020), and healthy LSD RASM, shares are trading closer to an appropriate multiple (of ~9x). This compares to history of 9-10x over the last several years and the group at ~8x, thus the airline is now better reflecting its premium margins and balance sheet relative to the long-run," Monday's note said. 

Delta wasn't the only airline to receive attention in Morgan Stanley's note. 

American Airlines (AAL) shares were cut to underweight from equal-weight, while the price target was slashed to $26, which is 16% below the company's opening price Monday of $30.65.

United Continental (UAL - Get Report) was upgraded to overweight from equal-weight while the price target was increased to $110 from $101.

"Over the last 12-18 months, United has done a solid job of executing on its midcontinent strategy, which we foresee continuing as the competitive backdrop remains benign in a high-cost environment," Morgan Stanley's note said. 

United shares were rising 0.8% to $82.31 on Monday.