I've been brushing up on my string theory using Becker, Becker, Schwarz (p. 208 in particular). They state that some D-branes can couple to R-R fields from closed string excitations.
They then go on to say that these branes are exactly 1/2 BPS and therefore stable. I understand that 1/2 BPS objects are stable because they preserve enough of the supersymmetry.
But I don't understand why the coupling to R-R fields ensures that the branes are 1/2 BPS.
Could someone possibly explain this point mathematically, and/or direct me to a reference that makes it clear?
Many thanks for your help!
Edit: perhaps the answer is contained within Polchinski's seminal paper here? Still might be useful to get a second opinion!