Thermal expansion of RFeAsO (R=La,Ce,Pr,Sm,Gd)

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 $T_{\mathrm{N}}$ . 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.