Grumpy_Mike:
From my former picture showing the wiring of my leds, I guessed it would be correct to call them common cathodes since they all share the ground wire. Now I know they shouldn't be called that... 
I've installed these in my ceiling planning to use them as ceiling lights. They give the right amount of light, and I am happy by the result driving them from my 15v power supply.
I know nothing about constant current driver circuits, but would it be possible to find/build a current driver that could drive two led-pcbs at once, as that's how I already wired them, instead of supplying each led-pcb with a driver?
Are there one such standard led current driver circuit, or do they vary a lot for these kind of leds? - I am thinking of which circuit to use to drive the led-pcbs.
Whoa.. I've got much to learn, constant current drivers are a whole new world to me... 
Best regards
Joe