Sequels to indie games Knytt Series and Really Big Sky are headed free to PlayStation Plus subscribers.

Indie publisher Ripstone has announced a trio of releases for Sony's PlayStation Plus Presents label: retro platformer Knytt Underground, hypnotic 'shmup Big Sky Infinity and a third, as-yet-unannounced game.

Both Knytt Underground and Big Sky Infinity will be playable on PlayStation 3 and PlayStation Vita.

All three will be freely downloadable to PlayStation Plus subscribers for eight weeks after launch.

PlayStation Plus Presents is the little brother to Sony's Instant Game Collection promotion. That service provides Plus gamers with a rotating roster of ten highly-rated mainstream games, downloadable for free. Recent highlights have included Dead Space 2 and Deus Ex: Human Revolution.