Here is the source: http://www.d20pfsrd.com/classes/base-classes/summoner/archetypes/paizo---su mmoner-archetypes/synthesist
While fused with his eidolon, the synthesist uses the eidolon’s physical ability scores (Strength, Dexterity, and Constitution), but retains his own mental ability scores (Intelligence, Wisdom, and Charisma). The synthesist gains the eidolon’s hit points as temporary hit points. When these hit points reach 0, the eidolon is killed and sent back to its home plane.
So, my Constitution score is exchanged for my eidolon's Constitution score, correct?
So... if I'm a level 1 Summoner with a 12 Con score:
1d8+1 = 9 hp
And my eidolon, with NO evolution points used, has a 13 Con score:
1d10+1 = 11 hp
I gain 2 TEMPORARY Hit points, so I now have a total of 11 HP. Is this correct?
Furthermore, if I lose those 11 hit points, my eidolon gets sent back to its home plane, but I retain my 9 ORIGINAL hit points, correct?
Thank you for your help.