Hello everybody and everyone!

No, no kidding. Perfectly serious. Though brass has been used for decades as a non-magnetic material.

I could observe by myself a piece of brass being attracted by a magnet.

The best conditions to see it was with a Nd-Fe-B magnet (of course) and with a small piece of brass rod, something like d=4mm and L=20mm rolling freely on a reasonably horizontal and smooth table. Then, the magnet would let the piece of rod roll on the table by attracting it from >5mm distance.

This observation was perfectly convincing. After having seen the rod roll, one could get the impression to feel directly with the fingers an attraction force between the magnet and the rod - but before, it wouldn't be clear enough, that's why I tried the rolling motion.

I confidently discarded eddy currents (would repel, not attract) as well as para- and diamagnetism (far too small).

So somehow brass, or at least the piece I had, must be slightly ferromagnetic. But then, how?

A colleague (Servus, Markus!) suggested the best explanation I got up to now: zinc ore always contains a bit of iron that would be incorporated in the brass, and after some heat treatments, this iron could precipitate as tiny crystals containing a high proportion of iron, so these tiny crystals could be ferromagnetic.

Any other suggestions?

Like your friend says, maybe the brass you have is not pure brass. I just picked up some magnets at a garage sale yesterday, and I can't resist the urge to try them on all sorts of things.

You most likely have Cu Ni. Here is a link
http://www.copper.org/applications/cuni/default.html