A ruling by the Ninth U.S. Circuit Court of Appeals appears to have thrown into doubt many of the approximately 100 mobile home rent control ordinances in effect in California cities and counties. A divided three-judge panel ruled that the City of Cotati's mobile home rent control ordinance was unconstitutional because, under the ordinance, the possibility existed that tenants might receive what amounts to a transfer of equity from landowners when tenants sell their mobile homes.