Linq to SQL Silverlight 4 WCF services

Cannot implicitly convert type 'System.Collections.Generic.List<Contract>' to 'System.Collections.Generic.List<string>'

public class Service : IService
{
    public List<string> GetContract(string Contract)
    {
        Contract_GenDataContext db = new Contract_GenDataContext();
        var matchingContract = from con in db.Contracts
                               where con.Contract1.Contains(Contract)
                               select con;
        return matchingContract.ToList();
    }
}