Before the accident that killed the crew, Selby and Chen worked in the ship's Catering Department, alongside Olaf Petersen (and presumably Tim). They spent all their free time drunk in ship's bars and disco, along with best friends Petersen and Lister. There, they played drinking games, such "I've Been to Titan" and wound Lister up about his romantic obsession with Kristine Kochanski, telling him that the Flight Officer was too classy for the Third Technician, and was way out of his league. Like Lister, Selby and Chen took great pleasure in deriding Arnold Rimmer and his salute. Lister, Petersen, Selby and Chen often ate breakfast together before Lister started his work on Z Shift. (RD: The End) (RD: Balance of Power) (RD: Stasis Leak) (Novels)
When the crew are resurrected by the nanobots at the beginning of Series VIII, Selby and Chen are the first of the resurrected crew the regular characters encountered. Selby and Chen were helping to put out the fire caused by the crashing Starbug in the hangar, and informed Lister that Petersen couldn't make it because he was drunk. After the regular characters are imprisoned in The Tank, Selby and Chen are not seen again. (RD: Back in the Red I)
Selby and Chen's appearance in "Back in the Red" is extended in the deleted scenes for the episode, where it is made clear that the third (still hooded) figure is indeed Olaf Petersen. As Thornton arrests Lister for stealing and crashing a Starbug, Selby and Chen say that they will stick by him, and get Lister the best lawyer money can buy. They then discuss going to the pub, and leave, quickly forgetting about Lister's plight.
Olaf Petersen, Selby and Chen, as officers presumably left Red Dwarf on the fleet of Starbugs and Blue Midgets to escape the Chameleonic Microbe along with the captain and the rest of the crew. (RD: Only the Good...) They are not seen again in Back to Earth or Series X.
Behind the scenes
Both David Gillespie (Selby) and Paul Bradley (Chen) had worked in British television and both had appeared in soap opera Eastenders together in the late 1980s.
Whilst Chen was clearly a chef, given his usual attire, Selby's profession was never clearly identified in the television series. The Red Dwarf novel Infinity Welcomes Careful Drivers goes into more depth about the characters, adding that Selby was also a catering officer along with Chen and Petersen. They were also the friends Lister went on a drunken pub crawl around London with, which ended up with Lister waking up on Mimas and initially joining Red Dwarf as a way to get back to Earth.