The Slips Marc Friedman and Andrew Barr will once again back politically charged folk-singer Dan Bern for a series of shows in the Northeast. The Slip rhythm section will perform as Berns backing band September 13 at Burlington, VTs Higher Ground, September 14 at Northampton, MAs Iron Horse and September 16 at New Yorks Irving Plaza. Slip guitarist Brad Barr will open all three nights and, perhaps, also join his bandmates onstage during Berns set. The Slip will appear Saturday at Japans True People’s Celebration.