Smeared branes and the Gubser-Mitra conjecture

We show that smeared brane solutions, where a charged black p-brane is smeared uniformly over one of the transverse directions, can have a Gregory-Laflamme type dynamical instability in the smeared direction even when the solution is locally thermodynamically stable. These thus provide counterexamples to the Gubser-Mitra conjecture, which links local dynamical and thermodynamic stability. The existence of a dynamical instability is demonstrated by exploiting an ansatz due to Harmark and Obers, which relates charged solutions to neutral ones.