[NTG-context] How to switch single font?

Dear list, how can I switch to another typeface for the, say, roman font only? When I use \definefontfamily to do so it switches *all* the fonts, including math, to the one loaded (example below). This leads to very undesirable effects, such as math is no longer set in italics. Cheers, Henri --- \definefontfamily[main][rm][Linux Libertine O] \setupbodyfont[main] \starttext foo $bar$ \stoptext