One of the all-time great R&B singers, Natalie Cole, has just joined the 2013 concert series, set to share a bill with Ramsey Lewis on Wednesday, Aug. 28. Donors can purchase tickets now through April 22. Tickets to the general public go on sale April 25. Cole has a new album expected on May 21. The concert begins at 7 p.m. with a performance by alumni from the jazz program at Ravinia’s Steans Music Institute. Dee Dee Bridgewater, who was originally scheduled to share the bill with Lewis, withdrew due to a scheduling conflict. Donors who have already purchased tickets to the Aug. 28 concert need do nothing.