Britain’s Eroica Quartet brings along four colleagues from Sir John Eliot Gardiner’s Orchestre Révolutionnaire et Romantique to give a period-instrument performance of Felix Mendelssohn’s masterly Octet, informed by the Library’s original score for the work. Monday, Oct. 19, at 8 p.m. University of Leeds musicologist Clive Brown gives a pre-concert lecture about the work at 6:15 p.m. The Whittall Pavilion at the Library of Congress, First & Independence Avenues SE. Tickets are free but required, available through Ticketmaster. Call 202-397-7328 or visit www.loc.gov/concerts.