CAIRO, Dec 18 (Reuters) - The Egyptian pound was unchanged at a central bank dollar sale on Thursday and weaker on the unofficial market after the government established a fourth weekly dollar auction.

The bank offered $40 million and said it had sold $38.2 million at a cut-off price of 7.1401 pounds to the dollar, a rate unchanged from its sales on Wednesday and Monday.

The rates at which banks are allowed to trade dollars are determined by the results of central bank sales, giving the bank effective control over official exchange rates.

In the unofficial market, the pound was trading at about 7.78 to the dollar on Thursday, weaker than Wednesday's rate of 7.70 to the dollar.

The price of dollars on the unofficial market had been largely steady since the government announced last week that it would hold a weekly Sunday auction in addition to the sales it already holds on Monday, Wednesday and Thursday.

The bank did not give a specific reason for adding the sale, but the central bank governor said last month he would take measures to calm the parallel market in the Egyptian pound, which has grown in recent weeks.

(Reporting by Maggie Fick; Editing by Crispian Balmer)