In right $ \triangle ABC$ with legs $ 5$ and $ 12$, arcs of circles are drawn, one with center $ A$ and radius $ 12$, the other with center $ B$ and radius $ 5$.  They intersect the hypotenuse at $ M$ and $ N$.  Then, $ MN$ has length:

[asy]size(200);
defaultpen(linewidth(0.7)+fontsize(10));
pair A=origin, B=(12,7), C=(12,0), M=12*dir(A--B), N=B+B.y*dir(B--A);
real r=degrees(B);
draw(A--B--C--cycle^^Arc(A,12,0,r)^^Arc(B,B.y,180+r,270));
pair point=incenter(A,B,C);
label("$A$", A, dir(point--A));
label("$B$", B, dir(point--B));
label("$C$", C, dir(point--C));
label("$M$", M, dir(point--M));
label("$N$", N, dir(point--N));

label("$12$", (6,0), S);
label("$5$", (12,3.5), E);[/asy]

$ \textbf{(A)}\ 2 \qquad \textbf{(B)}\ \frac {13}{5} \qquad \textbf{(C)}\ 3 \qquad \textbf{(D)}\ 4 \qquad \textbf{(E)}\ \frac {24}{5}$
1. **Identify the given information and the problem requirements:**
   - We have a right triangle \( \triangle ABC \) with legs \( AC = 5 \) and \( BC = 12 \).
   - The hypotenuse \( AB \) can be calculated using the Pythagorean theorem.
   - Two circles are drawn: one centered at \( A \) with radius \( 12 \) and the other centered at \( B \) with radius \( 5 \).
   - These circles intersect the hypotenuse \( AB \) at points \( M \) and \( N \).
   - We need to find the length of segment \( MN \).

2. **Calculate the hypotenuse \( AB \):**
   \[
   AB = \sqrt{AC^2 + BC^2} = \sqrt{5^2 + 12^2} = \sqrt{25 + 144} = \sqrt{169} = 13
   \]

3. **Determine the positions of points \( M \) and \( N \):**
   - Point \( M \) is where the circle centered at \( A \) with radius \( 12 \) intersects \( AB \).
   - Point \( N \) is where the circle centered at \( B \) with radius \( 5 \) intersects \( AB \).

4. **Use the properties of the circles to find \( M \) and \( N \):**
   - Since \( M \) is on the circle centered at \( A \) with radius \( 12 \), the distance \( AM = 12 \).
   - Since \( N \) is on the circle centered at \( B \) with radius \( 5 \), the distance \( BN = 5 \).

5. **Calculate the length of \( MN \):**
   - The total length of \( AB \) is \( 13 \).
   - The length \( MN \) can be found by subtracting the distances \( AM \) and \( BN \) from \( AB \):
   \[
   MN = AB - (AM + BN) = 13 - (12 + 5) = 13 - 17 = -4
   \]
   - However, this result is not possible since distances cannot be negative. The correct approach is to recognize that \( M \) and \( N \) are on the same line segment \( AB \), and the correct calculation should be:
   \[
   MN = AM + BN - AB = 12 + 5 - 13 = 4
   \]

The final answer is \( \boxed{4} \).