@8a3ca29f - things get really complicated!  In the Pythagorean scale here we get two different notes: the augmented 4th which is a bit lower than the tritone, and the diminished 5th which is a bit higher.  The augmented 4th is 

1024/729 ≈ 1.404664

the tritone is

√2 ≈ 1.414214

and the diminished 5th

729/512 ≈ 1.423828

I sure don't claim I can hear the difference!   But if two were played simultaneously they would sound out of tune.