Chrysanthemums are among my favorite perennials, so I'm always looking for fine asters to go with them.
The natives are nice, but I think 'Peter III', recently released by the folks at Yoder, is close to perfect. The soft blue color goes with everything, and Peter's' compact size is just right for snuggling in with mums.
Like his wild forbears, 'Peter III' blooms when the days get short in the fall, right on schedule with midseason mums.