It's interesting that I can't seem to find many non-danish non-recipe and non-gardening resources about oxheart cabbage.
I guess it is a rather Danish thing, even though I can only think of one traditional Danish recipe that uses it (in a stew).
It's pronounced ʁaʁyʁe.
And written RaRuRe, or R3 for short.
You can find more infos on this place by clicking there.