Part environment subtitle

[Piero Faustini]

Hello.
I use koma-book
Is there a way to set a subtitle for Part's title in a smaller font in a way 
that in the TOC then they're displayed in the same font?
What I would like is:

       Part I
     Big title
  smaller subtitle

and in TOC,

I. Big title - smaller subtitle

or

I. Big title. Smaller subtitle