  We consider transmission of discrete memoryless sources (DMSes) across
discrete memoryless channels (DMCs) using variable-length lossy source-channel
codes with feedback. The reliability function (optimum error exponent) is shown
to be equal to $\max\{0, B(1-R(D)/C)\},$ where $R(D)$ is the rate-distortion
function of the source, $B$ is the maximum relative entropy between output
distributions of the DMC, and $C$ is the Shannon capacity of the channel. We
show that, in this setting and in this asymptotic regime, separate
source-channel coding is, in fact, optimal.
