This article is from the Red Dwarf FAQ, by Patrick M. Berry pat@interpath.com with numerous contributions by others.
Throughout Series 6 and 7, the characters travel aboard Starbug; the Red
Dwarf and Holly don't appear at all. This tends to confuse viewers who
missed the explanation in "Psirens".
Briefly, what happened is this: Lister parked the Red Dwarf in orbit around
a planetoid, and then forgot which planetoid it was. "They're all the same,
those little blue-green planetoids. Blue-green and planetoidy."
Subsequently, Red Dwarf was stolen "by persons or lifeforms unknown."
Starbug followed the ship's vapor trail for over 200 years, with Lister and
the Cat in deep sleep and Rimmer switched off to conserve power. At the
first opportunity to actually catch up with Red Dwarf, Kryten revives the
others, and the saga continued . . .
The real reason for the change was to eliminate Holly from the show.
According to Hattie Hayridge, the parts for both Holly and the Cat had been
getting smaller. Grant Naylor decided to drop one of the characters and
expand the other. It was easier to drop Holly because many of her lines
could be given to Kryten instead.
In "Nanarchy," Starbug finally catches up to Red Dwarf, and Norman Lovett
returns to the show as Holly.
 
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