Old Valyria was super hot, and that was before the Doom

oh gods

Nope! The long vowel connects the two nouns

word "se" is used for "and" in second lesson

example:

kirimvose se geros ilas

thanks and bye

where:

kirimvose : thanks

geros ilas: bye

se: and

Short version: Check the bottom link. The last vowel in a set of two words is prolonged to indicate that they're coordinated (it prolongs the vowel instead of using 'and')

Long version: There's a feature in Duolingo, and it took me forever to discover. On the website, you can actually access a little written chapter for each lesson. In the first chapter of High Valyrian they describe pronounciation, coordination and other useful aspects.

To me, this is crucial information for understanding a language rather than memorising it. I haven't a clue why it's not accessible from the app.

Anyway, here is the first chapter for High Valyrian :) https://www.duolingo.com/skill/hv/Basics-1/tips-and-notes