Two vowels joined together
 

This one's been bugging me for a while. :)

In the UK and US there are many words we spell differently. A lot of these are (medical) words with double vowels eg:
oesophagus - esophagus
foetus - fetus
mediaeval - medieval
haemoglobin - hemoglobin

This is mostly a complete pain when I have to search US databases for medical information and remember to spell words differently.

I'd like to know the name of what I'm angry at. I don't think they're 'diphthongs' - or at least that's not specific enough, as 'ou' in 'loud' is a diphthong - it's any two vowels making one sound. My problem is usually 'ae' or 'oe', and when you see them written the letters are often squashed together (unlike 'loud').

So what's the word for what causes my problems?

Here's an interesting link. It looks like it is, in fact, a dipthong in some languages, but it's generally known as a "ligature". American English just seems to have dropped the old Greek convention.