* Fed meeting awaited, traders speculate on new stimulus * Sales of corporate debt weigh on Treasuries By Karen Brettell NEW YORK, Sept 11 (Reuters) - U.S. Treasuries prices slipped on Tuesday a day ahead of the start of a Federal Reserve meeting that is expected to result in a third round of bond purchases, and as investors prepared for $66 billion in new supply this week. Heavy sales of new corporate debt also weighed on Treasuries as investors hedged corporate bond purchases and also sold Treasuries to free up cash to purchase company debt. Treasuries have largely traded sideways within a small yield range this week, after a volatile trading session on Friday when weaker-than-expected jobs data increased expectations that the Fed would launch a third round of quantitative easing, dubbed QE3. "Bernanke's comments have been pretty bullish for QE; the main question is, What does that entail?" said Jason Rogan, director of Treasuries trading at Guggenheim Partners in New York. Fed Chairman Ben Bernanke will give a policy statement after the close of the U.S. central bank's two-day meeting on Thursday. Traders are speculating on details of any new program, including whether the U.S. central bank will announce open-ended purchases or commit only to buying on a month-to-month basis, and if purchases would extend to other assets including mortgage-backed debt. New Treasuries sales and heavy corporate debt supply also weighed on Treasuries yields ahead of Thursday's announcement. The U.S. Treasury Department will sell $32 billion in three-year notes later on Tuesday, followed by $21 billion in 10-year supply on Wednesday and $13 billion in 30-year bonds on Thursday. In the "when-issued" market, traders expect the upcoming three-year note issue due September 2015 to yield 0.338 percent , a hair above the 0.334 percent set a year ago, which was the lowest ever at a three-year note auction. Expectations that the Fed may extend its guidance to holding interest rates near zero percent through 2015 at this week's meeting is likely to help Tuesday's sale of three-year notes, traders said. The sale on Wednesday and Thursday of 10-year and 30-year bonds may be more complicated. "Not only is 30s normally a difficult auction but it's an hour in front of the Fed, so it will be interesting to see how the Street sets up for the auction," said Rogan. Heavy sales of corporate debt are also expected to extend into Tuesday, after a record day of issuance on Monday. Companies sold $18.95 billion in debt on Monday and may sell more than $10 billion on Tuesday, according to IFR, a Thomson Reuters service. In Europe, Germany's Constitutional Court said on Tuesday that it will go ahead with a long-awaited ruling on Wednesday on the legality of the euro zone's new permanent bailout fund and budget rules, despite a last-minute legal challenge by a member of parliament. A negative outcome, which most legal experts see as unlikely, would threaten the European Central Bank's plans to buy the bonds of struggling euro zone states to bring down their borrowing costs. Concerns that Spain was not moving closer to seeking aid from the region's rescue funds, a condition for the ECB bond purchases, capped some losses in Treasuries earlier on Tuesday. Spanish Prime Minister Mariano Rajoy said late on Monday he would not accept a rescue that dictated spending cuts.

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