  We present a comparison between the time-evolution over the past $\sim 20$
years of the radio continuum and H$\alpha$ emission of HH~1 and 2. We find that
the radio continuum and the H$\alpha$ emission of both objects show very
similar trends, with HH~1 becoming fainter and HH~2 brightening quite
considerably (about a factor of 2). We also find that the $F_{\rm
H\alpha}/F_{ff}$ (H$\alpha$ to free-free continuum) ratio of HH~1 and 2 has
higher values than the ones typically found in planetary nebulae (PNe) which we
interpret as an indication that the H$\alpha$ and free-free emission of HH~1/2
is produced in emitting regions with lower temperatures ($\sim 2000$~K) than
the emission of PNe (with $\sim 10^4$~K).
