Rachel Barton Pine not only plays violin, she also plays viola d'amore – an unusual instrument that generally resembles a violin, but has TWO sets of strings: one played directly with a bow, and the other that provides sympathetic vibrations. You’ll hear Rachel Barton Pine playing a Viola D’Amore Concerto by Antonio Vivaldi on Sunday Baroque this weekend, starting at 7 a.m. on 91.1, 107.5, and wshu.org, and at 10 a.m. on 89.9.