Shares in Weir Group were making decent gains on Monday morning after the stock was upgraded from 'neutral' to 'buy' at Citigroup, which said that the engineer's opportunities in oil and gas are "underappreciated" by the market.The bank said that "Weir deserves a premium rating to the sector to reflect positive exposure to structural trends in oil and gas, further earnings upgrade potential and improving returns".Credit Suisse has repeated its 'outperform' stance on diversified miner Rio Tinto, giving an optimistic outlook on cash generation and cost cutting. A 4,000p target price was maintained for the bank's "Focus List" stock.After a tour of Rio's North American operations, Credit Suisse said: "[the visit] revealed a relentless free-cash-flow (FCF) focus that goes well beyond the dominance of the Western Australian iron ore business. As a result, we see near-term upside to FCF generation and the $3bn/year cost cutting target."Credit Suisse has kept an 'underperform' rating on FTSE 250 gold miner Polymetal, saying that the company's recent acquisition of the Kyzyl gold project hasn't changed its cautious view.The investment case is "primarily driven by our negative gold price outlook", the bank said.Investec has upgraded pharmaceutical group Clinigen from 'hold' to 'buy', saying that even on a lower target price "we think the shares offer an attractive opportunity"."Clinigen's stock has fallen circa 40% since its interim results in March, when numbers were in line but growth was no longer as 'explosive' as it had been in prior years. We now think the shares are too cheap to ignore and, on balance, offer good value for investors."BC