We have the set of nonlinear equations: $$ \begin{cases} \dot x= \frac{r}{y-d}+y -x\\ \dot y = a+bx-y. \end{cases} $$ where parameters $r$, $d$, $a$, $b$ are real. There are two fixed points. If I had an autonomous system, I would evaluate the Jacobian in these fixed points in order to determine their stability. But this system is non-autonomous because $a$, and now I can't go on.
Some help with this would be much appreciated.