A cyclic quadrilateral $ABCD$ is given. An arbitrary circle passing through $C$ and $D$ meets $AC,BC$ at points $X,Y$ respectively. Find the locus of common points of circles $CAY$ and $CBX$.
1. **Define the Problem and Setup:**
   We are given a cyclic quadrilateral \(ABCD\). An arbitrary circle passing through \(C\) and \(D\) meets \(AC\) and \(BC\) at points \(X\) and \(Y\) respectively. We need to find the locus of the common points of circles \(CAY\) and \(CBX\).

2. **Identify Key Points and Lines:**
   Let \(O\) be the circumcenter of \(\triangle ABC\) and \(O_1\) be the circumcenter of \(\triangle CXY\). Let \(l\) be the perpendicular bisector of \(CD\).

3. **Perpendicular Bisectors and Intersection Points:**
   The perpendicular from \(O_1\) to \(AC\) and the perpendicular from \(O\) to \(BC\) intersect at a point \(P\). Similarly, define \(Q\) as the intersection of the perpendicular from \(O_1\) to \(BC\) and the perpendicular from \(O\) to \(AC\).

4. **Circumcenters and Reflections:**
   Notice that \(P\) and \(Q\) are the circumcenters of \(\triangle CBX\) and \(\triangle CAY\) respectively. Therefore, the other intersection point of the circles \(CAY\) and \(CBX\) is the reflection of \(C\) across the line \(PQ\).

5. **Homothety and Parallel Lines:**
   Let \(Z\) be the point at infinity on the line \(PQ\). As \(O_1\) moves along the line \(l\), the line \(PQ\) moves parallel to itself. This is due to the homothety centered at \(O\).

6. **Perpendicular from \(C\) to \(PQ\):**
   The desired point lies on the perpendicular from \(C\) to \(PQ\), which remains fixed. Let this line be denoted as \(k\).

7. **Cross Ratio and Conclusion:**
   We use the cross ratio to finalize our argument. The cross ratio \((-1) = (OP, OQ; OO_1, OZ) = (CB, CA; CD, k)\) holds because all corresponding lines are perpendicular, which does not change the cross ratio between them. Therefore, the line \(k\) is well-defined.

\[
\boxed{\text{The locus of the common points of circles } CAY \text{ and } CBX \text{ is the perpendicular from } C \text{ to } PQ.}
\]