  We consider two models of two-units repairable systems: cold standby system
and warm standby system. We suppose that the lifetimes and repair times of the
units are all independent exponentially distributed random variables. Using
stochastic orders we compare the lifetimes of systems under different
assumptions on the parameters of exponential distributions. We also consider a
cold standby system where the lifetimes and repair times of its units are not
necessarily exponentially distributed.
