Thanks a lot.
I'm not sure the "Dweller on the treshold" would apply to the spirits, and since magicians can dissimulate astral links in the metaplanes, it didn't seemed illogic, but Im pretty sure allowing spirits to sustain a power from their metaplane would be unbalanced, since a power can't be dispelled and isn't affected by astral wards, the only way to break it is destroying the spirit or knocking down a magician.
If I understand well, no need to sustain a mana spell cast on another astral from when going back to physical plane, but it can be done, okay, we'll care about the applications later.
Okay for the two sides of the magician, okay for sustaining a physical spell when going astral. Just a last check : can we still cast Mana spells on the physical body, or is it considered dead until the magician's astral form is back ?
I disagree only with this :
( as you must maintain LOS on the area you are sustaining the spell in).
I'm pretty sure you don't have to stay in LOS to sustain a spell with an area, but you have to stay in LOS to move the area.
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Yup, that's it. SR4a core rulebook, in spell casting, step 7. Cant' tell you the page since mine is in french, pages are not the same (unless you have a french version).