char
7     a    103 minutes
      s             63
9     a    129 minutes
      s            211
10    a    106 minutes
      s             63
Name:  timestamp, dtype: object

MultiIndex(levels=[[7, 9, 10], ['a', 's']],
           labels=[[0, 0, 1, 1, 2, 2], [0, 1, 0, 1, 0, 1]],
           names=['char', None])

a           s
char
7   01:43:00    00:00:00.000000
9   02:09:00    00:00:00.000000
10  01:46:00    00:00:00.000000