CAIRO, Aug 13 (Reuters) - The average yields on Egypt's three- and nine-month treasury bills were mixed at an auction on Sunday, central bank data showed, after falling sharply over the past week on the back of higher dollar reserves at the central bank.

The average yield on the 91-day bill fell to 19.080 percent from 19.131 at the last similar auction on August 6. The yield on the 266-day bill rose to 19.004 percent from 18.878 percent at the last auction.

Egypt's dollar liquidity has been improving since it floated its pound currency in November as part of a $12 billion International Monetary Fund programme aimed at boosting the economy.

Egypt's central bank said this month that foreign currency reserves jumped by $4.73 billion at the end of July to $36.04 billion, higher than before the 2011 uprising that drove away tourists and foreign investors, key sources of foreign currency.

(Reporting by Eric Knecht; Editing by Greg Mahlich) ((eric.knecht@thomsonreuters.com; +20 2 2394 8102; Reuters Messaging: eric.knecht.thomsonreuters.com@reuters.net))