I asked yesterday why the participle mortuus has two us. When Rafael asked whether one of the us were consonantal, I had no other evidence than being taught that they are both vocalic. Arguing by analogies is tricky as there seem to be no other verbs with the past participle stem ending in a vowel. This also makes it reasonable to conjecture that it might be /mortwus/ instead of /mortu.us/.
So, do we know how mortuus was pronounced in classical Latin? Did it have two or three syllables? Were the two adjacent us both vocalic? And most importantly, how do we know?