Oct 18 (Reuters) - Egypt's central bank kept the Egyptian pound steady against the dollar at Tuesday's weekly auction of foreign currency, while the national currency weakened on the black market.

The central bank sold $117 million at the regular dollar sale, with the cut off price stable at the official rate of 8.78 per dollar.

Two black market traders said they were selling dollars at 15.5 per dollar, higher than last week's range of 15 per dollar. They did not give volumes of trade.

Economists say the Egyptian pound is overvalued at its official rate and there is speculation the central bank will soon devalue to close the gap with black market rates.

(Reporting by Nadine Awadalla; Editing by Catherine Evans) ((Nadine.Awadalla@thomsonreuters.com;))