I've grown 2 of them from seed I started in February 2011.
On some sites they say this species doesn't go dormant..
But mine did it for the first time since about a month ago, I stopped watering them as the leaves were turning yellow, the bulbs look very healthy and are around 6cm (2,5inches) diameter.
I'm not sure now what to do..
The roots look healthy, firm and are still white. In dormancy, do I water just a little for the roots not to dry out, or do the roots die off too? do I have to re-pot them in fresh soil? Could the dormancy mean that the bulbs are ready to bloom when waking up?
When do I have to bring them out of their dormancy?
Sorry for so many questions but it is my first experience with growing Hippeastrum..

Myriam, I grow several Hippeastrum, but not H. papilio, so I can't say for certain. I do grow some that do not go dormant though, and they have gone through periods when the leaves have yellowed and the plants appeared to go dormant. During that time I did give them only a little water, but would say I kept them on the dry side. When they started sending up new leaves I started watering regularly. I would recommend that you do the same- if the bulb looks healthy and there are no leaves, the need for water is minimal. Those I grow that do go dormant seem to keep fleshy roots year round, so I believe yours will also keep white, firm roots.

Those I grow that go dormant are kept completely dry during dormancy. They start sprouting on their own, and that's what lets me know to start watering them.

Best of luck!

Thanks for sharing your knowledge Neal. That was very helpful!