  We present measurements of the thermal expansion coefficient $\alpha$ of
polycrystalline RFeAsO (R = La,Ce,Pr,Sm,Gd). Anomalies at the magnetic ordering
transitions indicate a significant magneto-elastic coupling and a negative
pressure dependence of $T_{\rm N}$ . The structural transitions are associated
by large anomalies in $\alpha$. Rare earth magnetic ordering in CeFeAsO,
PrFeAsO, and SmFeAsO yields large positive anomalies at low temperatures.
