Which fraction comes next in the sequence 23, 13, 527, 781, ?

Clearly, the numerators of the fractions in the given sequence follow the series of 2, 3, 5, 7,... and the denominator follows 3, 9, 27, 81, 243, ...
Then, the next term in the given sequence is   $\frac{9}{3} = \frac{9}{243} = \frac{1}{27}$ .