Citi analyst Kenneth Wong downgraded 3D Systems (DDD) to Sell from Neutral after the stock has nearly tripled since the market selloff in February. The recent hiring of Vyomesh Joshi as CEO is a positive but already priced into the shares at current levels, Wong tells investors in a research note. Investors believe Joshi's history at Hewlett-Packard enhance the possibility of an HP acquisition of 3D Systems, the analyst contends. He views a takeout as "highly unlikely," however. He raised his price target for 3D shares to $15 from $10. Wong this morning also downgraded Stratasys (SSYS) to Neutral on valuation grounds.