I called them R and S because one interacts only with reflecting things and one only with sensors. In the pub I had briefly tried calling them A and B, but found that made it far too hard to remember which one I'd called what.

Oddly enough, I did briefly have a completely different thought which was related to chirality: supposing vampires' invisibility in mirrors was fundamentally a consequence of the mirror reversing chirality, so that a vampire reflected in any odd number of mirrors was invisible but in an even number it was there again. I think that would need an entirely different explanation at the photon level, though, and I haven't sat down and worked one out. (Also, I admit to having blatantly pinched the chirality idea from China Miéville.)