  A polynomial $A(q)=\sum_{i=0}^n a_iq^i$ is said to be unimodal if $a_0\le
a_1\le \cdots \le a_k\ge a_{k+1} \ge \cdots \ge a_n$. We investigate the
unimodality of rational $q$-Catalan polynomials, which is defined to be
$C_{m,n}(q)= \frac{1}{[n+m]} \left[ m+n \atop n\right]$ for a coprime pair of
positive integers $(m,n)$. We conjecture that they are unimodal with respect to
parity, or equivalently, $(1+q)C_{m+n}(q)$ is unimodal. By using generating
functions and the constant term method, we verify our conjecture for $m\le 5$
in a straightforward way.
