Include subtitle in article toc

["Michael D. Cencula" <docbook@cencula.com>]

How can I create a stylesheet customization so that a section with
<title>my title</title>
<subtitle>my subtitle</subtitle>

gets rendered in the html article TOC as
my title - my subtitle

Basically I'm trying to include the subtitle in the table of contents 
following the title and a hyphen.

Thanks,

Mike Cencula