Find the missing number in the following series. $9, 7, (\text{?}), 9, 11, 11, 12, 13$

The series alternates between two interleaved sequences. Taking alternate terms: 9, (?), 11, 12 and 7, 9, 11, 13. The second sequence increases by 2: 7, 9, 11, 13. The first sequence increases by 1: 9, 10, 11, 12. Therefore the missing term is 10. This creates the pattern: 9, 7, 10, 9, 11, 11, 12, 13 where odd positions increase by 1 and even positions increase by 2.