On the minimum number of entries in a pair of maximal orthogonal partial Latin squares

It is shown that if $F$ denotes the number of filled cells in a superimposed pair of maximal orthogonal partial Latin squares of order $n$, then $F\ge n^2\mathrm{/}3$. This resolves a conjecture raised in an earlier paper by the current authors. It is also shown that, for $n\ge 21$, the least possible number of filled cells in a pair of maximal orthogonal partial Latin squares is $\lceil n^2\mathrm{/}3\rceil$, and that the structure that achieves this bound is unique up to permutations of rows, columns and entries.