Rails - Apr 2006 - create_table and char columns

When I use create_table to make a table :string always makes a varchar 
column. How can I make it so that :string will make a char column?

create_table(:accounts, :options => ''ENGINE=MyISAM
DEFAULT'') do |table|
  # This creates a varchar column, I would like a char column.
  table.column :username, :string
  ...
end

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