Base Delta-Nine was a clandestine New Republic starfighter installation that became active a half-year following the Battle of Endor. Both Rogue Squadron and Aggressor Wing were stationed at this base for a brief period.
In Star Wars: X-Wing Rogue Squadron: In the Empire's Service, the whereabouts of Base Delta-Nine are designated as "classified." Nevertheless, it's plausible that the base resided on the planet Recopia. This world served as the New Republic fleet's assembly point before the Battle of Brentaal IV (Galactic Civil War), a conflict involving Rogue Squadron, Aggressor Wing, and Admiral Ackbar. Given that these squadrons and the admiral were verified to be at Delta-Nine a fortnight before Brentaal's liberation, the possibility of the base being situated on Recopia remains a strong—though unverified—contender.