The form factors 
and 
have been discussed in all
kinds of models, Vector Meson Dominance, hard meson, etc.. For a
discussion see Ref. [9]. We will restrict ourselves to the
predictions in the framework of CHPT.
To leading order we have
We also have 
. The rate here is entirely given by the
inner Bremsstrahlung contribution. At the one-loop level
several form factors get non-zero values [11]
These results obey the current algebra relation of Ref. [9].
The function 
does, however, deviate somewhat from the
linear parametrization often used.
The function 
is defined in appendix B.
The fact that the form factors at
next-to-leading order could be written in terms of the kaon
electromagnetic form factor
in a simple way is not true anymore at the 
level.
The Lagrangian at order 
contains a term of the form
that contributes to 
and 
but not to the kaon electromagnetic
form factor, 
.
