  Fix a K3 lattice $\Lambda$ of rank two and $L\in\Lambda$ a big and nef
divisor that is positive enough. We prove that the generic $\Lambda$-polarised
K3 surface has an integral nodal rational curve in the linear system $|L|$, in
particular strengthening previous work of the first named author. The technique
is by degeneration, and also works for many lattices of higher rank.
