[FFmpeg-user] Which file formats can hold image subtitles (dvdsub) without other streams

[bbb]

Hi,

I need a file format for a temporary file to store image subtitles 
(dvdsub) in. So far I've been using .mpg which generally works, but 
seems to be unable to keep the metadata. When I try to use mkv it 
insists on having a video stream (... figures). Is there a file format 
in which I can temporarily store subtitles alone and keep their metadata?

Regards