Arsenal have signed French youth international striker Yaya Sanogo on a long-term contract, the London club said on their website (www.arsenal.com) on Monday.Sanogo, 20, a free agent after leaving Ligue 2 team Auxerre, has represented his country at Under-16, Under-17 and Under-19 level and is currently with the national side in Turkey for the Under-20 World Cup.He has scored in two of their three matches in Turkey, helping France qualify for the knockout phase.Sanogo had been with Auxerre since the age of 14. He made 24 first team appearances and scored 11 goals.