  We analyse the electromagnetic coupling in the Kemmer-Duffin-Petiau (KDP)
equation. Since the KDP--equation which describes spin-0 and spin-1 bosons is
of Dirac-type, we examine some analogies and differences from the Dirac
equation. The main difference to the Dirac equation is that the KDP equation
contains redundant components. We will show that as a result certain
interaction terms in the Hamilton form of the KDP equation do not have a
physical meaning and will not affect the calculation of physical observables.
We point out that a second order KDP equation derived by Kemmer as an analogy
to the second order Dirac equation is of limited physical applicability as (i)
it belongs to a class of second order equations which can be derived from the
original KDP equation and (ii) it lacks a back-transformation which would allow
one to obtain solutions of the KDP equation out of solutions of the second
order equation. We therefore suggest a different higher order equation which,
as far as the solutions for the wave functions are concerned, is equivalent to
the orginal first order KDP wave equation.
