DE-BUNDESWETTBEWERB 2006 Problem 7

A point $P$ is given inside an acute-angled triangle $ABC$ . Let $A',B',C'$ be the orthogonal projections of $P$ on sides $BC, CA, AB$ respectively. Determine the locus of points $P$ for which $\angle BAC = \angle B'A'C'$ and $\angle CBA = \angle C'B'A'$