1. The problem statement, all variables and given/known data Why is D*+(2010) → D+(1869)π0 allowed via the strong interaction but Ds*+(2110) → Ds+(1969)π0 forbidden. The numbers in the brackets are masses in MeV. 2. Relevant equations 3. The attempt at a solution My guess was that mass was not conserved, but if π0 is 135 MeV then it is. Strangeness is also conserved. Charge is also conserved. Isospin is also conserved. What else is there to consider?